Immunology (Lippincott Illustrated Reviews Series) 2nd Edition

Glossary

ABO blood groups

complex carbohydrate molecules most often found on the surfaces of erythrocytes. The ABO locus has three alleles: A, B, and O. The A and B alleles produce glycosyl transferases for N-acetylgalactosamine and galactose, respectively, whereas the O allele fails to produce a functional enzyme.

Acquired immune deficiency syndrome (AIDS)

a severe, often fatal human disease caused by a retrovirus that preferentially infects human CD4+ T lymphocytes and macrophages. Less than 40 years after its first clinical description, AIDS is now a worldwide pandemic. The long latency period (often >5 years) for disease onset contributes epidemiologically.

Activated lymphocytes

nongranular leukocytes stimulated specifically by antigen, nonspecifically by mitogen, or by cytokines often increase in size, a phenomenon known as blast transformation; causing the stimulated lymphocytes to perform effector function such as cytokine secretion.

Activated phagocytes

(e.g., macrophages, dendritic cells) increase phagocytic activity and size in response to a CYTOKINE signal from activated lymphocytes or in response to pathogen-associated molecular pattern (PAMP) molecules.

Activation

for leukocytes, the transformation from a resting state to a functionally active (effector) state. For molecules, the transformation from an enzymatically inactive to an enzymatically active state.

Active immunity

immunity resulting from sensitization of and response by an individual’s immune system, as distinguished from PASSIVE IMMUNITY.

Adaptive immunity

the ability of the immune system to acquire specific protection as the result of infection or intentional immunization.

Adherent cells

usually macrophages from lymphoid tissues or inflammatory exudates that adhere strongly to glass or plastic surfaces.

Adjuvant

any of many foreign materials injected along with an antigen to enhance its immunogenicity. These include killed bacteria (Bordetella pertussis, mycobacteria) or bacterial products (such as endotoxin) or emulsions (Freund’s complete adjuvant, alum).

Affinity

the thermodynamic quantification of the noncovalent interaction between two molecules, usually that of an antibody with its corresponding antigenic determinant, see also AVIDITY.

Agammaglobulinemia

severe deficiency or absence of one or more immunoglobulin isotypes.

Agglutination

clumping of particulate antigens (e.g., bacteria, cells, particles) as a consequence of cross-linking by antibody that serves to curtail microbial spread. As a technique, agglutination provides a simple and rapid means of determining blood groups such as ABO.

Allelic exclusion

at the cellular level, individual B lymphocytes produce immunoglobulin of only a single allelic form (i.e., ALLOTYPE) of light chain and heavy chain, different alleles, if present are not expressed. At the organismal level, alleles at all loci are expressed by different B cells.

Allergen

ANTIGEN, usually an IMMUNOGEN, that provokes ALLERGY.

Allergy

a type I HYPERSENSITIVITY response to an agent (i.e., ALLERGEN) that is nonantigenic for most individuals in a population. Most often, an antibody response of the IgE class is seen.

Allo-

prefix, meaning between genetically different members of the same species.

Alloantigen

an antigen obtained from another individual or inbred line of the same species. Such antigens are the result of genetic polymorphism. Histocompatibility molecules are common examples.

Allograft

tissue transplanted between genetically nonidentical individuals of the same species, also called homograft.

Allotype

a structural site on the heavy chain of a particular immunoglobulin class or on the light chain that differs among individuals or inbred lines of the same species. It can be used as a marker for genetic studies.

Alternative complement pathway

one of the three complement activation pathways. The alternative pathway is initiated by the binding of components C3 and B to certain PAMP produced by microbes.

Anamnestic response

same as SECONDARY IMMUNE RESPONSE.

Anaphylactoid reaction

nonimmunological local or systemic reaction often resulting from a physical stimulus such as trauma, heat, cold, etc.

Anaphylatoxin

small fragment of C3 or C5 (called C3a or C5a) that causes degranulation of mast cells and liberating vasoactive amines.

Anaphylaxis

systemic immediate hypersensitivity reaction resulting in respiratory distress or vascular collapse.

Anergy

immune nonresponsiveness (see suppression and TOLERANCE).

Antibody

immunoglobulin molecule capable of combining specifically with a known substance (antigen) (see IMMUNOGLOBULIN). The term antibody implies that the specificity is known.

Antibody-dependent cell-mediated cytotoxi city (ADCC)

the ability of nonsensitized cells (i.e., cells from an unimmunized animal) to lyse other cells that have been coated by specific antibody.

Antigen

molecule or part of a molecule recognized by the immune system. A specific target of the immune response. An antigen may be composed of many determinants or EPITOPES, see also DETERMINANT, EPITOPE, IMMUNOGEN.

Antigen-binding site

that portion of an antibody molecule that binds with the corresponding epitope, located in the Fab portion of the molecule.

Antigen presentation

display of antigenic peptide bound to MHC CLASS I or MHC CLASS II molecules on the cell surface. T LYMPHOCYTES recognize antigen only when presented in this manner.

Antigen processing

intracellular enzymatic degradation of antigen occurs prior to ANTIGEN PRESENTATION and is usually attributed to macrophages and dendritic cells.

Antigen suicide

method to specifically destroy cells carrying receptors for a particular antigen by exposing the cells to antigen of very high specific radioactivity. The localized irradiation leads to death of the cells. Alternatively, any toxic molecule attached to an antigen (e.g., ricin) that would lead to the destruction of a cell bearing a specific receptor for that antigen.

Antigenic determinant

minimum recognition unit of the immune response also known as an EPITOPE. It is generally believed to be as small as 4 to 6 amino acids.

Antigenicity

property of a substance permitting it to react with antibody or an antigen-specific T-CELL RECEPTOR, but an antigen does not necessarily induce an immune response, see also IMMUNOGEN.

Antihistamine

pharmacological substance (not an antibody) that blocks the effect of histamine.

Antiserum

the fluid, a cellular portion of the blood (serum) containing antibody molecules of known specificity. Antisera often are prepared by IMMUNIZATION with ANTIGENS.

Antitoxin

protective antibodies that inactivate soluble toxic proteins of bacteria.

Apoptosis

programmed cell death that follows a sequence of events including DNA cleavage, nuclear condensation, and plasma membrane blebbing leading to the phagocytosis of the apoptotic cell. Importantly, this type of cell death does not lead to an inflammatory response.

Appendix

a saclike structure arising from the cecum containing a rich accumulation of lymphoid cells.

Arthus reaction

local immune (hypersensitivity) reaction mediated by antigen–antibody complexes and resulting in vascular injury, thrombosis, hemorrhage, and acute inflammation.

Association

a statistical correlation between two events.

Association constant

or Ka value, the mathematical expression of affinity of binding between antigen and antibody.

Ataxia telangiectasia

a complex syndrome with neurological and immunological abnormalities. Ataxia—imbalance of muscular control. Telangiectasia—dilated capillary blood vessels.

Atopy

a genetically determined abnormal state of hypersensitivity, as distinguished from hypersensitivity responses in normal individuals.

Auto-Autoantibody

prefix, meaning within the same individual. an antibody produced by an individual directed against that individual’s own epitopes.

Autoimmune

immune response(s) directed against an individual’s own tissues, cells, or fluids.

Autoimmune disease

tissue injury and inflammation resulting from an autoreactive immune response that gives rise to clinically manifested anomalies and demonstrable histoimmunopathological changes.

Autologous

originating from the same individual.

Autoreactivity

immunologicactivity, eitherhumoralorcell-mediated, directed against host native or “self” antigenic component(s).

Autosomes

chromosomes other than the X and Y sex chromosomes.

Avidity

measure of binding strength of antibody and antigen molecules— usually involving multiple molecular interactions, see AFFINITY.

B

symbol for a component of the ALTERNATIVE COMPLEMENT PATHWAY, also known as C3 proactivator of C3PA

B cell

one of the two major classes of lymphocytes. B cells, derive from the BURSA OF FABRICIUS in birds or the BONE MARROW of mammals, respond to an immunogenic signal by differentiating into antibody-producing cells (i.e., plasma cells).

B-1 cell

a self-renewing class of B lymphocytes most often found in the peritoneal and plural cavities.

B-2 cell

conventional B lymphocytes.

Basophil

a white blood cell of the granulocytic series (see MAST CELL) that has receptors for the Fc portion of homocytotrophic or heterocytotrophic antibodies. Important in the allergic response.

Bence-Jones protein

protein found in urine consisting of immunoglobulin light chains or their fragments—frequently because of the presence of malignant overgrowth by one clone of lymphocytes or plasma cells (e.g., multiple myeloma).

β2 Microglobulin

a 12,000-Da polypeptide, whose amino acid sequence shows homology with an immunoglobulin heavy chain domain, found in association with histocompatibility antigens on the surface of cells.

Binding site

that portion of a receptor (e.g., T-CELL RECEPTOR or IMMUNOGLOBULIN molecule) that displays a significant AFFINITY for a ligand.

Blood

the circulating tissue of the body, it is composed of a pale yellow fluid called plasma, erythrocytes (red cells), leukocytes (white blood cells), and cellular fragments (platelets).

Blood groups

surface molecules on red blood cells that may vary between individuals of the same species. The most important blood groups in man are the ABO and Rh blood groups.

Bone marrow

hemopoietic tissue, precursors (stem cells) of most of the cellular elements of the blood are located here. Lymphocytes differentiating at this site are referred to as B CELLS, and cells of this lineage give rise to immunoglobulins.

Booster

secondary challenge with antigen.

Bruton-type agammaglobulinemia

sex-linked genetic inability to form B cells and hence immunoglobulins.

Buffy coat

the “skin” of leukocytes that forms above the red cell layer when whole blood is allowed to settle or pelleted by centrifugation.

Bursa of Fabricius

hindgut lymphoid organ in birds that influences B CELL development. BONE MARROW is the mammalian equivalent.

Bystander

a cell or tissue not actively involved at the site of an immune reaction, but close proximity to that reaction makes it subject to damage as a consequence of an immune reaction.

C3 Receptor

(C3R) a site on the surface of B cells and phagocytes able to bind activated C3.

Capping

the coordinated surface movement of membrane molecules to one region of the cell surface after binding by a multivalent ligand such as an antibody or an antigen.

Caspases

Intracellular cysteine proteases involved in apoptotic death pathways.

CD

cluster of differentiation.

Cell-mediated immunity (CMI)

immune responses mediated by cells. Includes CYTOTOXIC T LYMPHOCYTE REACTIVITY, DELAYED HYPERSENSITIVITY, ANTIBODY-DEPENDENT CELL-MEDIATED CYTOTOXICITY, and in general, any cell-mediated immunological effector function.

Cellular immunity

see CELL-MEDIATED IMMUNITY.

Central lymphoid organs

lymphoid tissues that serve as the differentiation sites for lymphocytes. In mammals, these are THYMUS (T CELLS) and BONE MARROW (B CELLS). Birds have a defined organ for Bcell differentiation called the BURSA OF FABRICIUS.

CH region

segment of the heavy chain of immunoglobulin with a relatively constant amino acid sequence. There are several of these regions of homology or DOMAINS on the heavy chain that are sequentially labeled CH1, CH2, CH3 (sometimes CH4).

Chaperone

a molecule controlling the three-dimensional folding and transport of another molecule.

Chediak–Higashi syndrome

disease based on faulty phagocytic destruction of ingested microbes and related to lysosomal membrane abnormalities.

Chemokine

a member of a large family of low molecular weight cytokines that stimulates lymphocyte movement and migration from the circulation into the tissues.

Chemotactic factor for macrophages

produced by TH1 CELLS, provokes migration of macrophages to the site of cell-mediated immune reactions.

Chemotaxis

a process whereby phagocytic cells are attracted to the vicinity of invading pathogens.

Cis

the arrangement of linked, nonallelic genes located on the same member of the homologous chromosome pair (as opposed to trans).

CL region

the region of the light chain of immunoglobulins with a relatively constant amino acid sequence in different antibodies.

Class (antibody)

the major molecular types of immunoglobulin: IgM, IgG, IgA, IgE, IgD; see also ISOTYPE.

Class I molecules

the classical transplantation antigens or MHC MOLECULES. Glycoproteins of ~45,000 Da. Products of HLA-A, HLA-B, HLA-C loci.

Class II molecules

products of HLA-D/DR regions. These are noncovalently associated heterodimers consisting of α(~33,000 Da) and β(~28,000 Da) chains, see also Ia ANTIGENS.

Class III molecules

complement proteins (C2, C4, factor B) encoded by genes within the major histocompatibility complex.

Classical complement pathway

the mechanism of complement activation by antigen–antibody complexes involving the binding of C1, C4, C2 to activate C3.

Clonal anergy

a theory stating that the interaction of B cells with antigen may lead to the selective inactivation of specific B cells resulting in tolerance during ontogeny.

Clonal deletion

concept that tolerance results from the elimination (i.e., deletion) of self-reactive clones.

Clonal restriction

the ability of a lineage of cells to react only upon stimulation with a ligand in the context of a particular MHC MOLECULE.

Clonal selection theory

hypothesis explaining the specific nature of the immune response in which the diversity among various cells for the recognition of specific antigens exists prior to their exposure to the antigen. Subsequent exposure to a particular antigen causes the proliferation of the appropriate antigen-specific cells.

Clone

a group of genotypically and phenotypically identical cells, all of which are the descendants of a single cell.

Cluster of differentiation (CD)

term used to serologically identify lymphoid cell surface molecules as detected by different monoclonal or polyclonal antibodies.

Colony-stimulating factor

molecules that stimulate the growth of cells.

Colostrum

first milk secreted by mother after birth.

Complement

a set of serum proteins activated in sequence by antibody–antigen complexes or by bacterial products (ALTERNATIVE COMPLEMENT PATHWAY) and responsible for many biological defense mechanisms such as lysis, opsonization, leukocyte chemotaxis, inflammation, etc.

Complement fixation

the binding of COMPLEMENT to an antigen–antibody complex.

Complement fixation test

a genetic test for determining whether two variants involve the same locus or chromosomal segment.

Congenital

existing at birth, may be hereditary trait or because of some other influence arising during gestation.

Constant region

that region of an immunoglobulin chain with a close sequence homology to other chains of that class or subclass.

Contact dermatitis

delayed or cell-mediated hypersensitivity response to cutaneously applied immunogens.

Contact sensitivity

form of DELAYED HYPERSENSITIVITY in which sensitivity to topically applied simple chemical compounds is manifested by a skin reaction.

Cortex

the peripheral region of the lymph node or thymus.

Cross-link

the joining of two similar or dissimilar molecules or cells by a chemical by covalent or noncovalent means.

-cyte

Suffix, meaning cell (e.g., splenocyte or spleen cell).

Cytokines

Protein molecules that act as messengers between cells affecting their behavior.

Cytotoxic T lymphocytes

lymphocytes (T CELLS) that have been sensitized and are able, by direct contact, to specifically lyse target cells to which they bind, see CML.

D

symbol for a component of the ALTERNATIVE COMPLEMENT PATHWAY.

Death domain

portions of protein molecules involved in the apoptotic death pathway. Their definition has now been expanded to include other protein–protein interactions.

Degranulation

a process whereby cytoplasmic granules of phagocytic cells fuse with phagosomes and discharge their contents into the phagolysosome thus formed.

Delayed (-type) hypersensitivity

specific inflammatory immune reactions elicited by antigen in the skin of immune individuals. Takes 24 to 48 hours to develop and is mediated by T cells and macrophages, but not by antibodies.

Deletion

loss of a section of genetic material from a chromosome.

Dendritic cell

specialized antigen-presenting cells, the most potent stimulator of T-cell responses.

Desensitization

the reduction of abolition of allergic reactivity. This term is really a misnomer, because, in fact, the patient is actively immunized with the allergen in hopes of producing an IgG response that will supplant IgE.

Determinant

that part of the structure of an antigen or immunogen that binds to the antibody combining site of an immunoglobulin or that part of an antigen or immunogen specifically recognized by the T-cell receptor, see EPITOPE, hapten.

Dextran

high-molecular-weight glucose polymers. Often, these are T-independent antigens.

Diapedesis

the outward passage of cells through the intact vessel walls.

Differentiation antigen

a serologically detectable cell-surface antigen expressed at a particular stage of differentiation.

DiGeorge syndrome

birth defects in embryonic development of the thymus resulting in loss of immune competence requiring T lymphocytes.

Discriminate

to tell the difference between two antigens and especially between “self” and “nonself” (foreign) antigens, a fundamental property of the immune system.

Diversity

the large number of antigen-specific receptors produced by the immune system.

Domain

a single homology region of an immunoglobulin, encompassing about 110 amino acids, and held together by a disulfide bridge spanning the central 60 or so residues.

Dominant

an allele whose phenotypic effect is evident whether in the homozygous or heterozygous state.

Edema

accumulation of fluid in a tissue.

Effector cell

a cell focused on influencing other cells either by the production of cytokines, other regulatory molecules/processes, or with the purpose of destroying other cells.

Endocytosis

internalization extracellular molecules or particles by pinocytosis or phagocytosis.

Endogenous

having origin within the organism.

Endotoxin

a lipopolysaccharide derived from cell walls of gramnegative— has multiple biological effects—stimulates the immune response nonspecifically, stimulates mouse B lymphocytes, and activates the alternative complement pathway.

Enzyme-linked immunosorbent assay (ELISA)

an immunoassay employing an enzyme covalently bound to either antibody or antigen as a marker. A chromogenic substrate is used to detect the presence of enzyme and hence antibody or antigen.

Eosinophil

a leukocyte with a bilobed nucleus and red-staining cytoplasmic granules, often found at sites of parasitic infections.

Epistasis

a form of gene interaction in which expression of one gene is dependent on the expression or activity of another nonallelic gene(s).

Epitope

a single antigenic determinant—the portion of a molecule that will combine with a particular antibody combining site. Multiple epitopes usually found on the same antigen, see ANTIGENIC DETERMINANT.

Equivalence

a ratio of antigen–antibody concentration where maximal precipitation occurs.

Erythroblastosis fetalis

medical term for Rh incompatibility disease of the newborn.

Extravasation

movement of the fluids and/or cells from the blood vessels into the surrounding tissue.

Exudation

discharge of plasma into a tissue.

Fab fragment

a product of papain digestion of immunoglobulins, contains one intact light chain and part of one heavy chain. Fab fragments have one combining site for antigen.

F(ab)2 fragment

a product of pepsin digestion of immunoglobulins, containing two intact light chains and parts of two heavy chains. It has two combining sites for antigen, but lacks the Fc region.

Fas (CD95)

member of the TNF receptor family expressed on the cell surface. Engagement of Fas triggers apoptosis of the Fasbearing cell.

Fc fragment

a product of papain digestion of immunoglobulin with parts of two heavy chains and no combining sites for antigen. This fragment has sites for activation of complement. It contains no antigen-binding capacity, but determines important biological characteristics of the intact molecule.

FcR

see FC RECEPTOR.

Fc receptor

a receptor for the Fc portion of the immunoglobulin molecule; present on various subclasses of lymphocytes.

Fd fragment

N-Terminus, papain cleavage fragment of the heavy chain of an immunoglobulin molecule. This fragment linked by disulfide bond to a light chain forms a Fab fragment.

Fluorescein (FITC)

an organic molecule that absorbs light at 485 nm and emits light at 535 nm. This property is exploited immunological in that the isothiocyanate form of this molecule (FITC) is used to react with a and e amines as a convenient label for both antibodies and antigens.

Fluorescence

the emission of light of one color while a substance is irradiated with a different wavelength.

Follicle

a circumscribed region in lymphoid tissue, usually in the superficial cortex of lymph nodes, containing mostly B cells.

Foreign

A chemical, organism, or substance not naturally found in your body, something that triggers the immune system to make a protective response.

Gamma (γ) globulin

γ Globulins with slow electrophoretic mobility in the γ region, including most immunoglobulin molecules. This term is sometimes used to refer to all immunoglobulins of various classes or isotypes.

Gammopathy

disorder involving abnormalities of immunoglobulins.

Gastrointestinal associated lymphoid tissue (GALT)

those accumulations of lymphoid tissue (e.g., tonsils, appendix, Peyer’s patches, and in lamina propria) that are responsible for monitoring and protection of the gastrointestinal tract.

Gene

a distinctive hereditary unit located on a chromosome at a specific site or locus that codes for a functional product (e.g., tRNA, rRNA, or a polypeptide chain).

Generation of diversity

the generation of a vast diversity of antibodies and T-cell receptors to recognize some 106 to 107different antigens.

Germinal centers

a collection of metabolically active lymphoblasts, macrophages, and plasma cells that appear within the primary follicle of lymphoid tissues following antigenic stimulation.

Germline

the genetic lineage of an individual or cell.

Germline theory

the explanation proposed for receptor diversity in which all of the genes responsible for all the possible receptors are fully formed and transmissible within the genome.

Graft rejection

an immune reaction elicited by the grafting of genetically dissimilar tissue onto a recipient. The reaction leads to destruction and ultimate rejection of the transplanted tissue.

Graft-versus-host reaction (GVH)

the pathological reactions caused by transplantation of immunocompetent T lymphocytes into an incompetent host. The host is unable to reject the T lymphocytes and becomes the target of their attack.

Granuloma

a local accumulation of densely packed macrophages, often fusing to form giant cells, and sometimes including lymphocytes and plasma cells. Seen in chronic infections such as tuberculosis and syphilis.

Graves disease

autoimmune disease in which antibodies are produced against thyroid-stimulating hormone receptor, stimulating thyroid hormone secretion, and hyperthyroidism.

Growth factors

soluble molecules that promote the growth or proliferation of a particular cell or tissue.

H2 or H-2

the MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) of the mouse.

H2K, H2D, H2L

MHC class I loci of the mouse. They are the equivalent of the HLA-A, -B, and -C loci in the human MHC.

Hay fever

a seasonal allergic disease causing inflammation of the eyes and nasal passages.

Heavy chain

the higher molecular weight polypeptide chain of an immunoglobulin molecule—the one determining the class or isotype of the immunoglobulin.

Helper T cells

a class of specific CD4+ T lymphocytes that are necessary to “help” B lymphocytes produce antibody to thymic-dependent immunogens and effector T cells to carry out their respective functions.

Hemagglutination

see AGGLUTINATION.

Hemagglutination inhibition

a technique for detecting small amounts of antigen in which homologous antigen inhibits the agglutination of red cells or other particles coated with antigen by specific antibody.

Heterologous

from a different individual, inbred strain, or species. Sometimes applied to a different carrier molecule.

Heterophil antigen

an antigen found on seemingly unrelated organisms, cells, or molecules.

High endothelial venules (HEV)

specialized vessels that allow the passage of circulating lymphoid cells into the parenchyma.

Hinge region

that portion of the immunoglobulin molecule lying between the Fc and Fab portions; so named because of the “flexibility” of this region of the molecule, most likely because of proline residues in this region.

Histamine

an amine found in all plant and animal tissues, it causes vasodilation and hence, lowers blood pressure.

Histiocytes

fixed or immobile macrophages.

Histocompatibility antigen

a genetically encoded molecule that can be recognized by the immune system of another individual of the same species.

Histocompatibility gene

a gene encoding a histocompatibility molecule (antigen).

Histocompatible

the ability to transplant tissues between individuals without rejection.

HLA-A, HLA-B, HLA-C

three distinct genetic loci in the human MHC encoding class I major histocompatibility antigens. Equivalent of the H2K, H2D, and H2L loci in the mouse H2 complex.

HLA-D/DR

a region of the human HLA complex encoding class II major histocompatibility complex antigens expressed primarily on B cells and macrophages and that stimulate the specific proliferation of allogeneic T cells in culture. Equivalent to the I-region in the mouse H2 complex.

Homocytotropic antibody

antibody that attaches to mast cells of the species producing it; IgE is an example.

Human immunodeficiency virus (HIV)

a retrovirus that causes human acquired immunodeficiency syndrome (AIDS).

Human leukocyte antigen (HLA) complex

the major histocompatibility complex of humans.

Humor or Humoral

pertaining to the fluids or humors of the body. In immunology, this term refers antibodies and/or complement. According to the ancient Greeks, the body is governed by the four humors; phlegm, blood, choler (or yellow bile), and black bile.

Humoral immunity

immune responses involving specific antibody, complement, and/or other soluble molecules.

Hybridoma

a hybrid cell derived in the laboratory from the somatic cell fusion of a normal B or T cell with a tumor cell line of either B cell (myeloma) or T cell (thymoma) origin, respectively. Such hybrid cells have the ability to produce antibody (B cell) or T cell products (T cell) of the normal “parent” and are immortalized by the tumor cell “parent.”

Hyperimmunization

heightened immune response resulting from repeated immunization with the same antigen.

Hyperplasia

physical increase in size of an organ or tissue because of an increase in cell number.

Hypersensitivity

a poor term, yet widely used, which is usually applied to those immune phenomena that are in some way damaging to the host animal.

Hypervariable region

defined portions of the variable region of either heavy or light immunoglobulin chains having extreme variability in amino acid sequence in different molecules. The antibodycombining site includes the hypervariable regions.

Idiopathic

primary disease or pathological state arising from uncertain causes.

Idiotype

an antigenic determinant on a specific antibody, characteristic of that antibody, and different from others even of the same isotype and ALLOTYPE—idiotypes are usually located in or near the antigen-binding site, see EPITOPE.

Immediate hypersensitivity

a specific immune reaction taking place within minutes to hours after the administration of antigen and is mediated by antibodies, see ARTHUS REACTION, ANAPHYLAXIS.

Immune adherence

adhesive nature of antigen–antibody complexes to inert surfaces when complement is bound into the complex.

Immune complexes

antigen–antibody complexes.

Immune deficiency

a defect in immune responsiveness because of environmental insult or genetic factors.

Immunity

an active process performed by leukocytes and their products in the repulsion of a foreign organism or substance.

Immunization

the process of inducing a state of immunity. This can result from the conscious introduction of immunogen (antigen) into the body as in VACCINATION or can result from the introduction of a foreign organism such as a microorganism to the immune system.

Immunoabsorption

removal of particulate or soluble substance from solution by antibodies specific to these substances. Often, the antibodies are bound to a solid matrix such a beaded agarose or are precipitated from solution by chemical means.

Immunodeficiency

genetic absence of a cell type or tissue or the acquired inability of the immune system to respond to a particular stimulus or stimuli.

Immunoelectrophoresis

a technique combining an initial electrophoretic separation of proteins with an immunodiffusion resulting in precipitation arcs.

Immunofluorescence

a histochemical or cytochemical technique for the detection and localization of antigens in which specific antibody is conjugated with fluorescent compounds, resulting in a sensitive tracer that can be detected by fluorometric measurements.

Immunogen

a substance that, when introduced into an animal, stimulates an immune response and also serves as the target of that response.

Immunoglobulin

the various classes of γ-globulin molecules having antibody activity.

Immunologic memory

the primary tenet of the immune system, allowing a rapid response to subsequent exposure to the same or related immunological stimulus.

Immune cell adhesion molecules (ICAMs)

ligands for leukocyte integrins crucial for the binding of leukocytes to other cells (e.g., antigen-presenting cells and endothelial cells).

Immunological unresponsiveness or tolerance

inhibition, by any one of several mechanisms, of activation and proliferation of an immunocompetent clone of lymphoid cells.

Inflammation

redness, swelling, and pain in a tissue resulting from the infiltration of the tissue by infectious agents and/or lymphoid cells.

Inflammatory cells

T lymphocytes (Th1 and CTL) and monocytes or macrophages that mediate and inflammatory response.

Innate immunity

the mechanical (e.g., skin), chemical (e.g., pH), and biological (e.g., symbiotic flora of the gut, phagocytic cells) components that provide natural barriers to infectious agents.

Inoculation

introduction of a substance into the body or into a culture.

Interferon -α, -β, or -γ

a LYMPHOKINE showing nonspecific activity in providing protection against viral infection and tumor growth. Three varieties of IFN produced by lymphocytes and NATURAL KILLER (NK) cells.

Interleukin

a term applied to any of a group of peptides signals that are produced by activated lymphocytes or monocytes.

Interleukin-1 (IL-1)

or lymphocyte-activating factor. Produced by macrophages and other cells, promoting multiplication and activation of B cells and T cells.

Interleukin-2 (IL-2)

formerly called T-cell growth factor. Produced by CD4+ T cells. Induces and maintains clonal growth of activated T cells.

Interleukin-4 (IL-4)

formerly called B-cell growth factor, produced by CD4+ T cells. It synergizes with IL-1 in B-cell activation.

Isotype

the class or subclass of an immunoglobulin.

Isotype switching

the ability of B cells to alter the class or isotype of antibody produced without significantly altering antibody specificity.

J chain

a small polypeptide found in IgM and IgA polymers—responsible for maintaining the polymeric form of the immunoglobulin.

Janus kinases (JAK)

intracellular tyrosine kinase activated during cytokine signaling.

Junctional diversity

diversity created by the process of joining V, D, and J gene segments.

Kappa (κ) chain

immunoglobulin light chain, one of two types known to exist, see LAMBDA CHAIN. Classification is based on the amino acid sequence of the constant portion of the light chain.

Killer cell or K cell

a class of null lymphocytes able to mediate ADCC. Most likely these are NK CELLS.

Lambda (λ) chain

an immunoglobulin light chain, one of two types known to exist (see KAPPA CHAIN). Classification is based on the amino acid sequence of the constant portion of the light chain.

Langerhans cell

epidermal dendritic cells.

Late-phase reaction

antihistamine resistant type I hypersensitivity reaction.

Lectin

any of several plant products that bind to cells, usually by means of a combining site for specific sugars.

Leukocytes

circulating white blood cells. There are about 9,000/mm3 in human blood, divided into granulocytes (polymorphs 68% to 70%, eosinophils 3%, and basophils 0.5%) and mononuclear cells (monocytes 4% and lymphocytes 23% to 25%).

Ligand

any molecule that forms a complex with another molecule.

Light chain

the lower molecular weight polypeptide chain present in all immunoglobulin molecules, see also LAMBDA CHAIN, KAPPA CHAIN.

Lipopolysaccharide (LPS)

active component of endotoxin, derived from bacterial cell walls, a B-cell mitogen in the mouse.

Locus

a position on a chromosome at which the genes for a particular trait are found.

Lupus erythematosus

fatal autoimmune disease characterized by the production of antinuclear antibodies, see also SLE.

Lyme disease

immune response to a chronic infection caused by the spirochete, Borrelia burgdorferi, often with the hallmarks of rheumatoid arthritis.

Lymph fluid

the acellular serous exudate from capillaries picked up by the lymphatic drainage vessels and thus circulated throughout the lymphatic network.

Lymphatics

vessels of the immune system that drain interstitial tissues of fluids, debris, and leukocytes.

Lymphocyte

a white blood cell of the lymphoid series, the “workhorse” cell of the immune system. It is capable of recognizing and responding to antigens in a specific manner.

Lymphoid organs

accumulations of lymphoid cells into regular anatomical structure (e.g., spleen, thymus, lymph nodes, appendix).

Lymphokine-activated killer (LAK) cell

a cytotoxic leukocyte requiring a lymphokine as one of its inductive signals.

Lymphokines

a group of substances produced by lymphocytes having diverse effects on other cells.

Lymphoma

a cancer of the lymphoid organs.

Lymphotoxin

a lymphokine (tumor necrosis factor-β, TNF-β) that directly causes cytolysis. It is released from stimulated lymphocytes.

Lysosome

cytoplasmic organelle present in many cells, bounded by a lipoprotein membrane, which contains various enzymes. Plays an important role in intracellular digestion.

Lysozyme

enzyme(s), glycosidases, present in the granules of polymorphs, in macrophages, in tears, mucus, and saliva. They lyse certain bacteria, especially gram-positive cocci, by splitting the muramic acid and (1-4)-N-acetylglucosamine linkage in the bacteria cell wall, thereby potentiating the action of complement on these bacteria.

Macrophage

a phagocytic cell found in tissues and blood. When found in blood, it is called a monocyte. It is sometimes referred to as a histiocyte.

Major histocompatibility complex (MHC)

a region of genetic material containing genes coding for certain predominant histocompatibility antigens, immune response and suppression loci, some lymphocyte and macrophage antigens, and complement components. Three classes of MHC molecules are recognized. Class I molecules are single chain, ~45,000 Da molecules that associate with ~2 microglobulin. Class II molecules are heterodimers termed α (alpha) and β (beta) chains (~29,000 and ~33,000 Da, respectively) that are not covalently linked. Class III MHC molecules are complement proteins whose genetic loci lie within the MHC. In man, the MHC is called HLA, in mouse H2, and in rat Rt-1.

Mannan-binding lectin (MBL)

an acute phase protein that binds to mannose residues and can activate the complement system and is important in innate immunity.

Mast cell

a leukocyte of the granulocytic series that mediates anaphylactic reactions. Mast cells have been shown to bear Fc receptors for anaphylactic antibody (IgE). Interaction of IgE antibodies on the surface of mast cells with antigen results in the degranulation of the cell and the release of vasoactive amines (i.e., histamine, heparin, etc.).

Medulla

the central region of a lymph node or the thymus, consisting of lymphatic sinuses and medullary cords.

Megakaryocyte

multinuclear giant cell of the bone marrow, portions of which break off to form the platelets.

Membrane attack complex (MAC)

those components of the terminal pathway of complement Images that forms a stable macromolecular complex serving as the building block for the final and lytic component of complement (C9).

Memory

the ability of the immune system to mount a specific secondary response to an immunogen that was previously introduced.

Memory cell

a lymphocyte, either B or T, that has undergone the first stages of differentiation as a consequence of have been specifically stimulated by immunogen.

MHC class II transactivator (CIITA)

a protein that activates the transcription of MHC class II genes.

Microglia

phagocytic cells of the nervous system; some investigators report the expression of MHC class II molecules by these cells.

Microorganism

microscopic organisms that include bacteria, fungi, and protozoans.

Migration inhibition factor (MIF)

a protein produced by lymphocytes upon interaction with antigens. It inhibits mobility of macrophages in culture.

Mitogen

a substance that induces mitosis in lymphocytes causing them to proliferate independently of any specific immunogen.

Mitogen-activated protein (MAP) kinases

proteins that are phosphorylated and activated upon cellular stimulation by various ligands.

Mixed lymphocyte (or leukocytereaction (MLR) or culture (MLC)

the in vitro proliferation of T lymphocytes as the result of “recognition” of foreign antigen on another lymphocyte or monocyte.

Modulation

temporal variation in the expression of a particular alloantigen on the surface of a cell or population of cells.

Molecular mimicry

induction of an immune response to an infectious organism, the consequence of which is a cross-reactive immune response to self-antigens that may have close structural similarities to molecules of the infectious agent.

Monoclonal

derived from a single clone of cells. Recently, this has come into use to describe antibodies made from a HYBRIDOMA.

Monoclonal antibody

antibody originating from cells having a single-cell precursor as its origin. Often, this is applied to HYBRIDOMA-derived antibodies but is also applicable to paraprotein antibodies derived from myeloma (B-cell tumor) cells.

Monocyte

a phagocytic blood leukocyte, precursor of most tissue macrophages. Monocytes originate from cells in the bone marrow.

Monokine

soluble factors released by activated macrophages/monocytes.

Monomer

a single polypeptide chain.

Mononuclear cells

leukocytes with a single, nonlobed nucleus; included in this group are monocytes and lymphocytes.

Mucosal cell adhesion molecule-1 (MadCAM-1)

the ligand for lymphocyte surface proteins L-selectin and VLA-4 that allow the specific homing of lymphocytes to mucosal tissues.

Multiple myeloma

a disorder typically consisting of the presence of serum paraprotein, anemia, and lytic bone lesions.

Multiple sclerosis

a presumed autoimmune neurological disease that results from the demyelination of proteins.

Myasthenia gravis

an autoimmune disease involving the production of antibodies against acetylcholine receptor on skeletal muscle leading to progressive weakness and death.

Myeloblast, myelocyte

an immature cell derived from the bone marrow that gives rise to cells of the polymorphonuclear series.

Myeloma

or plasmacytoma; a cancer of immunoglobulin-secreting cells.

Naive lymphocytes

cells that have never encountered specific antigen.

Natural immunity

immunity conferred without known sensitization.

Natural killer (NK) cell

type of NULL CELL involved in destruction of tumor cells thought to be one of the cells responsible for IMMUNE SURVEILLANCE.

Necrosis

death of cell or tissue because of chemical or physical injury, as opposed to APOPTOSIS.

Negative selection

deletion of thymocytes that recognize self during intrathymic development.

Neoantigens

spontaneously arising nonself antigens found on cell membranes, usually associated with neoplasia.

Neutralization

the process by which antibody and complement neutralizes the activity of microorganisms, particularly viruses or soluble substances such as toxins.

Neutralizing antibodies

specific immunoglobulins that inhibit the infectivity of a virus or the toxicity of a molecule.

Neutrophil

the most prominent leukocyte in the circulation, accounting for over 90% of the circulating granulocytes or 60% to 70% of the circulating leukocytes. A pinocytotic cell involved in complement-mediated and DTH reactions. An increase in the number of circulating neutrophils indicates chronic infection.

NK cell

see NATURAL KILLER CELL

Nuclear factor of activated T cells (NFAT)

a transcription factor that upon activation (serine/threonine dephosphorylation and dissociation from the Fos/Jun dimer, AP-1) moves from the cytoplasm to the nucleus.

Null cell

a class of lymphocytes without markers for either T cells or B cells.

Oncogene

a gene involved in regulating cell growth; defect structure or function of these genes leads to the continuous growth of cell forming a tumor.

Ontogeny

the developmental history of an individual organism within a group of animals.

Opportunistic infection

infection caused by a microorganism in individuals with a compromised immune system.

Opsonin

any substance that enhances phagocytosis of a cell or particle. Antibodies appear to be the only opsonin occurring normally in the body.

Opsonization

enhancement of phagocytosis of a particle or a cell (especially bacteria) by virtue of its being coated by antibody.

Original antigenic sin

a secondary immune response elicited with an immunogen that is not identical to the priming antigen. The ensuing immune response is directed more strongly with the priming rather than the secondary immunogen.

Paraprotein

immunoglobulin or immunoglobulin light chain produced as a consequence of the excessive clonal expansion of plasma cells, see BENCE-JONES PROTEIN.

Parasite

an organism that obtains sustenance from a live host.

Paratope

the antigen-binding site of an antibody molecule.

Passive immunity

immunity transferred to an individual with serum or immune cells as distinguished from ACTIVE IMMUNITY.

Patching

aggregation of membrane molecules into many small regions on the cell surface after cross-linking by a multivalent ligand such as antibody.

Pathology

study of disease mechanisms.

Perforin

A cytolytic protein produced by NK and cytotoxic cells, important in cell-mediated killing.

Peripheral lymphoid tissues

spleen, lymph nodes, tonsils, Peyer’s patches, etc. These are lymphoid accumulations in which an antigen-driven immune response can occur, seeCENTRAL LYMPHOID ORGAN.

Peyer’s patches

collections of lymphoid tissue in the submucosa of the small intestine that contain lymphocytes, plasma cells, germinal centers, and T-dependent areas.

Phagocytes

cells that can ingest particulate matter.

Phagocytosis

the engulfment of microorganisms or other particulate matter by phagocytic cells.

Pharyngeal pouch

ectodermal embryological structure occurring as blind sacs in the cervical region of the embryo, give rise to the epithelial tissues of the thymus, parathyroids, etc.

Phenotype

characteristic of an individual or cells of that individual that reflect the genes expressed by that individual or cell.

Pinocytosis

the ingestion of soluble materials by cells.

Plasma

the fluid phase of whole blood, containing water, salts, proteins, and clotting factors.

Plasma cell

a fully differentiated cell of the B-lymphocyte lineage, actively secreting large amounts of immunoglobulin.

Platelet or thrombocyte

a cell fragment deriving from a prothrombocyte, responsible for the activation of the clotting mechanism.

Platelet-activating factors (PAFs)

substances released immunologically and able to aggregate and degranulate platelets.

Poison ivy

a plant that elicits contact sensitivity to the pentadecacatechol present in its leaves.

Pokeweed mitogen (PWM)

a lectin derived from pokeweed that stimulates both B and T lymphocytes in man.

Polyclonal

immunologically, arising from cells of differing antigenic specificities.

Postcapillary venules

small vessels found downstream of the capillary bed through whose walls lymphoid cells are often found to migrate.

pre-B cells

immature cells developmentally destined to become B CELLS, in contrast to B cells pre-B cells express immunoglobulin only in their cytoplasm.

Precipitation

a reaction between a soluble antigen and soluble antibody in which a complex lattice of interlocking aggregates forms and falls out of solution.

Precipitin

the insoluble aggregate formed from the interaction of soluble antigen with soluble antibody, see also PRECIPITATION.

Prednisone

anti-inflammatory synthetic steroid.

Presentation

the display of small peptide fragments by specialized proteins on the surface of antigen-presenting cells or virusinfected cells.

Primary follicles

tightly packed accumulations of lymphoid cells (primarily B cells) in spleen and thymus, destined to become germinal centers.

Primary immune response

response occurring upon first exposure to an immunogen.

Primed

refers to an animal or cell population that has been previously exposed to an immunogen and that is capable of making a secondary response.

Privileged sites

anatomical site thought to be exempt from normal immune monitoring.

Processing

uptake and breakdown of antigen by host accessory cells leading to the presentation of antigen in an immunogenic form. Dendritic cells, macrophages, and to a lesser extent B cells, may process antigens and present them to lymphocytes in association with self Ia molecules.

Professional antigenpresenting cells

cells that initiate response of naive T cells (e.g., dendritic cells, macrophages, and B cells).

Progenitors

cells that give rise to distinct subsets of mature blood cells.

Programmed cell death

apoptosis, cell death triggered from within the cell.

Properdin

a component of the ALTERNATIVE COMPLEMENT PATHWAY.

Properdin pathway

see ALTERNATIVE COMPLEMENT PATHWAY.

Prostaglandins

aliphatic acids with a wide variety of biological activities including vasodilation and smooth muscle contraction.

Proteasome

a large protein complex involved in the degradation of intracellular proteins.

Protective immunity

resistance from specific infection because of prior infection or immunization.

Protein A

a protein derived from the Cowan strain of Staphylococcus aureus that has binding affinity for the FC portion of several different immunoglobulin isotypes.

Prothymocyte

a precursor to the thymus cells, the embryological origin is the bone marrow in mammals and the yolk sac in birds.

Pus

a fluid product of inflammation containing leukocytes.

Pyrogens

substances often derived from bacteria (e.g., endotoxins) that cause a characteristic rise in body temperature of an individual.

Radioallergosorbent test (RAST)

a radioimmunoassay that measures the amount of serum IgE antibody bound to a specific allergen or immunogen.

Radioimmunoassay (RIA)

a test that measures radiolabeled ligand (or antibody) binding to an antibody (or ligand).

Receptor

immunologically, protein structures that are either soluble or within the membranes of cells that interact with ligands.

Recessive

an allele whose phenotypic effect is evident only when present in the homozygous state.

Recombination activating gene (RAG)

encode proteins RAG-1 and RAG-2 that are critical to receptor gene rearrangement.

Rejection

unsuccessful transplant because of an immune reaction (both cellular and humoral) against the transplanted tissue.

Repertoire

the entirety of antigens that are recognized by an individual’s immune system.

Respiratory burst

the metabolic change in neutrophils and macrophages that occurs following phagocytosis of opsonized particles.

Reticuloendothelial system

a system of cells that take up particles and certain dyes injected into the body. Composed of Kupffer cells of the liver, tissue histiocytes, monocytes, and the lymph node, splenic, alveolar, peritoneal, and pleural macrophages.

Rhesus (Rh) system

a system of human red cell protein antigens under complex genetic control. Rh-negative mothers who bear Rhpositive offspring may develop Rh antibodies, which can cross the placenta and produce hemolytic disease in newborn babies.

Rheumatoid factor (RF)

an anti-immunoglobulin antibody directed against denatured IgG present in the serum of patients with rheumatoid arthritis and other rheumatoid diseases.

S value

sedimentation constant of 1 × 10−13, usually used as a measure of relative protein size and determined by centrifugation.

Secondary follicle

a GERMINAL CENTER.

Secondary immune response

the response occurring on the second and subsequent exposures to an immunogen (memory). The secondary immune response is usually characterized by a much more rapid immune response than is seen in a primary reaction.

Second set rejection

rapid graft rejection because of prior sensitization of the graft recipient.

Secretory component

see TRANSPORT PIECE.

Secretory IgA

a dimer of IgA molecules with a sedimentation coefficient of 11S, linked by the J chain and secretory component.

Secretory piece

a 70,000 Da molecule produced in epithelial cells and associated with secretory immunoglobulins (e.g., IgA), also called TRANSPORT PIECE.

Selectins

leukocyte cell-surface adhesion molecules that bind to specific glycoproteins and mucin-like molecules.

Self-tolerance

failure to respond to antigen expressed by self-tissues.

Sensitization

exposure to an individual or cell to an immunogenic form of antigen.

Sensitized cell

a cell that has been exposed to a specific stimulatory signal (e.g., exposed to immunogen).

Sepsis

bloodstream infection.

Seroconversion

first appearance of antibodies in the blood against a particular infectious agent.

Serologically defined (SD) determinant

a determinant defined by serological methodology. Sometimes used to refer to class I alloantigens of the MHC, although they can also be defined by cellular methods.

Serology

literally, the study of serum.

Serum

the liquid portion of coagulated (clotted) blood remaining after cells and fibrin (“clot”) have been removed (usually by centrifugation).

Serum sickness

systemic syndrome resulting from the deposition of circulating immune complexes, leading to complement-mediated inflammation in blood vessels and glomeruli of the kidney.

Severe combined immunodeficiency (SCID)

a genetically recessive stem cell deficiency affecting both T and B lymphocytes; may be because of autosomal or X-linked genes.

Sib

sibling, one or more offspring of the same parents.

Specificity

the ability of antibodies and T lymphocytes to distinguish between different determinants (epitopes). Also used to refer to a specific determinant.

Spleen

the ductless vascular lymphoid organ located in the upper left quadrant of the abdomen divisible into white pulp (lymphoid cell rich) and red pulp (erythrocyte rich) regions.

Splenomegaly

increase in spleen size. Often used as an assay for graftversus- host reactions.

Src family tyrosine kinases

receptor-associated protein tyrosine kinases, have domains termed Src-homology (SH) domains-1, -2, -3 or SH1, SH2, SH3.

Stem cell

a multipotent precursor cell that may give rise to cells of different morphological and functional specificities.

Subclass

immunoglobulins of the same class (e.g., IgG), but differing in electrophoretic mobility or in an antigenic determinant detectable in the CH region (e.g., IgG1, IgG2, IgG3, IgG4).

Superantigen

molecule that stimulates T-cell subsets by binding to MHC class II molecules and the Vβ domain of the T-cell receptor.

Suppressor T cells

a class of T lymphocytes able to suppress the immune response to an antigen. There are specific and nonspecific suppressor T cells.

Systemic lupus erythematosus (SLE)

an autoimmune disease characterized by the production of autoantibodies to different autoantigens and especially to DNA.

T cell

a class of lymphocytes derived from the thymus and able to respond to thymic dependent antigens and major histocompatibility complex gene products. T cells do not produce antibodies. They mediate cellular reactions, “help” B cells, and regulate responses.

T-cell receptor

a heterodimer molecule consisting of α and β chains on most T cells and γ and δ chains on the rest.

T-cell-replacing factor (TRF)

a factor produced by CD4+ T cells that induces B-cell differentiation.

TAP-associate protein

tapasin, a key molecule in the assembly of MHC class I molecules.

Target cells

cells that are subject to “attack” by effector cells.

Tat

protein product of the tat gene of HIV, binds to a transcriptional enhancer of the long terminal repeat of the provirus thus increasing transcription.

Template theory

proposed that lymphocytes use antigen as a mold for the construction of a receptor. Theory discarded when each lymphocyte was found to be inherently antigen specific.

Th1 cells

a subset of CD4+ T cells characterized by the cytokine they produce. Often involved in cell-mediated immune responses.

Th2 cells

a subset of CD4 cells characterized by the cytokines they produce. Often involved in stimulating B cells to produce antibody, see also HELPER T CELLS.

Thymocyte

a lymphocyte resident in the thymus, generally considered to be maturing functionally, once the cell leaves the thymus it is called a T CELL.

Thymus

a central lymphoid organ, the site of T-cell development.

Thymus-dependent area

region within peripheral lymphoid tissue containing mostly B cells that does not atrophy after thymectomy (e.g., the follicle of lymph nodes and spleen found in the superficial cortex).

Titer

a term used to connote the relative strength of an antiserum. An antiserum is progressively diluted until some measurable property of the antiserum (agglutination, facilitation of complement mediated lysis, etc.) is reduced by some predetermined amount. That dilution (e.g., 1:256) is then defined as the titer for that particular antiserum.

Tolerance

failure of the immune system, as the result of previous contact with antigen, to respond to the same antigen on subsequent occasions, although able to respond to others. Tolerance is best established by neonatal injection of antigen.

Tolerogen

a substance that preferentially induces tolerance.

Tonsils

any collection of lymphoid tissue, in particular those lymphoid accumulations surrounding the pharynx (pharyngeal and palatine tonsils).

Toxic shock syndrome

a systemic toxic reaction to Staphylococcus aureus resulting in the massive production of cytokines by CD4+ T cells.

Toxin

a poisonous substance that is either the intracellular or extracellular part of the cell or tissue.

Toxoid

a toxin treated in a manner that inactivates or removes its toxic activity.

Trans

linked, nonallelic genes located on different members of a homologous chromosome pair (as opposed to cis).

Transduction

the transfer of a genetic fragment from one cell to another, especially the transfer of bacterial genes from one bacterium to another by a bacteriophage.

Transfection

insertion of segments of DNA into cells; a stable transfection results when the DNA fragment integrates into host DNA, if not, it is called a transient transfection.

Transplantation antigen

a histocompatibility antigen.

Transport piece

a polypeptide found in association with secreted IgA, but not with serum IgA, also called SECRETORY COMPONENT.

Transporters associated with antigen processing (TAP-1, TAP-2)

ATP-binding cassette proteins involved in the transport of short peptides from the cytosol to the lumen of the endoplasmic reticulum for loading into MHC class I molecules.

Tuberculin test

a clinical test in which purified protein derivative (PPD) is injected subcutaneously to elicit a delayed hypersensitivity reaction; a positive reaction indicates that the individual has been infected by or has been immunized against Mycobacterium tuberculosis.

Tumor necrosis factor-α (TNF-α)

a cytokine produced by macrophages and T cells with multiple functions.

Tumor-specific transplantation antigens (TSTA or TSA)

antigens found on the membranes of tumor cells, but not on normal cells from the same or identical individuals, against which immunological reactions are directed.

Urticaria

or hives, red welts usually because of an allergic reaction.

Vaccination

inoculation of a nonvirulent or inactivated virus or bacterium as a means of inducing specific immunity. The term derives from vaccinia or the cowpox virus, used by Edward Jenner to induce a specific immunity to smallpox.

Vaccinia

or cowpox virus, causes limited infection in humans, but leads to immunity to smallpox virus.

Valence

term applied to antibody molecules indicates the number of epitopes to which an antibody molecule can bind. IgG, IgE, IgD, and IgA have a valency of 2, secretory IgA has a valency of 4, and IgM has a valency of 10.

Variable region

that portion of an immunoglobulin molecule or of the T-cell receptor that conveys antigen specificity to the molecule. By its very name, the amino acid sequence of one molecule varies as compared to another molecule of differing specificity.

Vasculitis

inflammation of a lymphatic or blood vessel.

Vasoconstriction

narrowing of the blood vessels, often because of the contraction of smooth muscle fibers.

Vasodilation

temporary enlargement of the lumen of a blood vessel.

VH region

the variable amino acid sequence region of the heavy chain.

VL region

the variable amino acid sequence region of the light chain.

Wheal

an acute swelling of a circumscribed area of the skin as the result of edema of the skin.

Wheal-and-flare reaction

raised area of the skin, the result of an allergic reaction to dermally injected allergen.

White blood cell

a LEUKOCYTE.

White pulp

the leukocyte-rich area of the spleen.

Wild type

the allelic form of a gene most frequently found in nature, which is arbitrarily designated as “normal.”

Wiskott–Aldrich syndrome

sex-linked genetic disease with combined losses of B and T lymphocytes, especially affecting IgM production.

Xeno-

prefix, between species.

Xenograft

exchange of tissue between members of different species.

Zymosan

a preparation of yeast cell walls that activates the alternative pathway of complement.