Review of Medical Microbiology and Immunology, 13th Edition

63. Complement



Activation of Complement

Regulation of the Complement System

Biologic Effects of Complement





Enhancement of Antibody Production

Clinical Aspects of Complement

Self-Assessment Questions

Practice Questions: USMLE & Course Examinations


The complement system consists of approximately 20 proteins that are present in normal human (and other animal) serum. The term complement refers to the ability of these proteins to complement (i.e., augment) the effects of other components of the immune system (e.g., antibody). Complement is an important component of our innate host defenses.

There are three main effects of complement: (1) lysis of cells such as bacteria, allografts, and tumor cells; (2) generation of mediators that participate in inflammation and attract neutrophils; and (3) opsonization (i.e., enhancement of phagocytosis). Complement proteins are synthesized mainly by the liver.


Several complement components are proenzymes, which must be cleaved to form active enzymes. Activation of the complement system can be initiated either by antigen–antibody complexes or by a variety of nonimmunologic molecules (e.g., endotoxin).

Sequential activation of complement components (Figure 63–1) occurs via one of three pathways: the classic pathway, the lectin pathway, and the alternative pathway (see later). Of these pathways, the lectin and the alternative pathways are more important the first time we are infected by a microorganism because the antibody required to trigger the classic pathway is not present. The lectin pathway and the alternative pathway are, therefore, participants in the innate arm of the immune system.


FIGURE 63–1 The classic and alternative pathways of the complement system. Images indicates that proteolytic cleavage of the molecule at the tip of the arrow has occurred; a line over a complex indicates that it is enzymatically active. Note that all small fragments are labeled “a,” and all large fragments are labeled “b.” Hence the C3 convertase is depicted as C4b,2b. Note that proteases associated with the mannan-binding lectin cleave C4 as well as C2.

All three pathways lead to the production of C3b, the central molecule of the complement cascade. The presence of C3b on the surface of a microbe marks it as foreign and targets it for destruction. C3b has two important functions: (1) It combines with other complement components to generate C5 convertase, the enzyme that leads to the production of the membrane attack complex; and (2) it opsonizes bacteria because phagocytes have receptors for C3b on their surface.

(1) In the classic pathway, antigen–antibody complexes1 activate C12 to form a protease, which cleaves C2 and C4 to form a C4b,2b complex. The latter is C3 convertase, which cleaves C3 molecules into two fragments, C3a and C3b. C3a, an anaphylatoxin, is discussed later. C3b forms a complex with C4b,2b, producing a new enzyme, C5 convertase (C4b,2b,3b), which cleaves C5 to form C5a and C5b. C5a is an anaphylatoxin and a chemotactic factor (see later). C5b binds to C6 and C7 to form a complex that interacts with C8 and C9 to produce the membrane attack complex (C5b,6,7,8,9), which causes cytolysis. Note that the “b” fragment continues in the main pathway, whereas the “a” fragment is split off and has other activities.

(2) In the lectin pathway, mannan-binding lectin (MBL) (also known as mannose-binding protein) binds to the surface of microbes bearing mannan (a polymer of the sugar, mannose). This activates proteases associated with MBL that cleave C2 and C4 components of complement and activate the classic pathway. Note that this process bypasses the antibody-requiring step and so is protective early in infection before antibody is formed.

(3) In the alternative pathway, many unrelated cell surface substances (e.g., bacterial lipopolysaccharides [endotoxin], fungal cell walls, and viral envelopes) can initiate the process by binding C3(H2O) and factor B. This complex is cleaved by a protease, factor D, to produce C3b, Bb. This acts as a C3 convertase to generate more C3b.


The first regulatory step in the classic pathway is at the level of the antibody itself. The complement-binding site on the heavy chain of IgM and IgG is unavailable to the C1 component of complement if antigen is not bound to these antibodies. This means that complement is not activated by IgM and IgG despite being present in the blood at all times. However, when antigen binds to its specific antibody, a conformational shift occurs and the C1 component can bind and initiate the cascade.

Several serum proteins regulate the complement system at different stages.

(1) C1 inhibitor is an important regulator of the classic pathway. It inactivates the protease activity of C1. Activation of the classic pathway proceeds past this point by generating sufficient C1 to overwhelm the inhibitor.

(2) Regulation of the alternative pathway is mediated by the binding of factor H to C3b and cleavage of this complex by factor I, a protease. This reduces the amount of C5 convertase available. The alternative pathway can proceed past this regulatory point if sufficient C3b attaches to cell membranes. Attachment of C3b to cell membranes protects it from degradation by factors H and I. Another component that enhances activation of the alternative pathway is properdin, which protects C3b and stabilizes the C3 convertase.

(3) Protection of human cells from lysis by the membrane attack complex of complement is mediated by decay-accelerating factor (DAF, CD55)—a glycoprotein located on the surface of human cells. DAF acts by binding to C3b and C4b and limiting the formation of C3 convertase and C5 convertase. This prevents the formation of the membrane attack complex.



Microbes, such as bacteria and viruses, are phagocytized much better in the presence of C3b because there are C3b receptors on the surface of many phagocytes.


C5a and the C5,6,7 complex attract neutrophils. They migrate especially well toward C5a. C5a also enhances the adhesiveness of neutrophils to the endothelium.


C3a, C4a, and C5a cause degranulation of mast cells with release of mediators (e.g., histamine), leading to increased vascular permeability and smooth muscle contraction, especially contraction of the bronchioles leading to bronchospasm. Anaphylatoxins can also bind directly to smooth muscle cells of the bronchioles and cause bronchospasm. C5a is, by far, the most potent of these anaphylatoxins. Anaphylaxis caused by these complement components is less common than anaphylaxis caused by type I (IgE-mediated) hypersensitivity (see Chapter 65).


Insertion of the C5b,6,7,8,9 complex into the cell membrane forms a “pore” in the membrane. This opening in the membrane results in the killing (lysis) of many types of cells, including erythrocytes, bacteria, and tumor cells. Cytolysis is not an enzymatic process; rather, it appears that insertion of the complex results in disruption of the membrane and the entry of water and electrolytes into the cell.

Enhancement of Antibody Production

The binding of C3b to its receptors on the surface of activated B cells greatly enhances antibody production compared with that by B cells that are activated by antigen alone. The clinical importance of this is that patients who are deficient in C3b produce significantly less antibody than do those with normal amounts of C3b. The low concentration of both antibody and C3b significantly impairs host defenses, resulting in multiple, severe pyogenic infections.


(1) Inherited (or acquired) deficiency of some complement components, especially C5–C8, greatly enhances susceptibility to Neisseria bacteremia and other infections. A deficiency of MBL also predisposes to severe Neisseria infections. A deficiency of C3 leads to severe, recurrent pyogenic sinus and respiratory tract infections.

(2) Inherited deficiency of C1 esterase inhibitor results in angioedema. When the amount of inhibitor is reduced, an overproduction of esterase occurs. This leads to an increase in anaphylatoxins, which cause capillary permeability and edema.

(3) Acquired deficiency of decay-accelerating factor on the surface of cells results in an increase in complement-mediated hemolysis. Clinically, this appears as the disorder paroxysmal nocturnal hemoglobinuria (see Chapter 68).

(4) In transfusion mismatches (e.g., when type A blood is given by mistake to a person who has type B blood), antibody to the A antigen in the recipient binds to A antigen on the donor red cells, complement is activated, and large amounts of anaphylatoxins and membrane attack complexes are generated. The anaphylatoxins cause shock, and the membrane attack complexes cause red cell hemolysis.

(5) Immune complexes bind complement, and thus complement levels are low in immune complex diseases (e.g., acute glomerulonephritis and systemic lupus erythematosus). Binding (activating) complement attracts polymorphonuclear leukocytes, which release enzymes that damage tissue.

(6) Patients with severe liver disease (e.g., alcoholic cirrhosis or chronic hepatitis B), who have lost significant liver function and therefore cannot synthesize sufficient complement proteins, are predisposed to infections caused by pyogenic bacteria.


1. Regarding the complement pathway, which one of the following is the most accurate?

(A) C3 convertase protects normal cells from lysis by complement.

(B) C3a is a decay-accelerating factor and causes the rapid decay and death of bacteria.

(C) In general, gram-positive bacteria are more likely to be killed by complement than gram-negative bacteria.

(D) The membrane attack complex is formed as a result of activation of the classic pathway but not by activation of the alternative pathway.

(E) The first time a person is exposed to a microorganism, the alternative pathway of complement is more likely to be activated than the classic pathway.

2. Of the following complement components, which one is the most important opsonin?

(A) C1

(B) C3a

(C) C3b

(D) C5a

(E) C5b

3. Of the following complement components, which one is the most potent in attracting neutrophils to the site of infection (i.e., acting as a chemokine)?

(A) C1

(B) C2

(C) C3b

(D) C5a

(E) Mannan-binding lectin

4. Of the following, which one is the most important function of the complex formed by complement components C5b,6,7,8,9?

(A) To enhance antibody production

(B) To inhibit immune complex formation

(C) To opsonize viruses

(D) To perforate bacterial cell membranes

(E) To release histamine from mast cells

5. A deficiency of which one of the following complement components predisposes to bacteremia caused by members of the genus Neisseria?

(A) C1

(B) C3b

(C) C5a

(D) C5b

(E) C5b,6,7,8,9

6. Your patient is a 20-year-old woman who complains of swellings on her arms and legs and a feeling of fullness in her throat that makes it difficult to breath. The swellings are not red, hot, or tender. You suspect she may have angioedema caused by a complement abnormality. Of the following, which one is the most likely explanation?

(A) She has too little C1 inhibitor.

(B) She has too little C3b.

(C) She has too little factor B.

(D) She has too much C5a.

(E) She has too much C9.


1. (E)

2. (C)

3. (D)

4. (D)

5. (E)

6. (A)


Questions on the topics discussed in this chapter can be found in the Immunology section of PART XIII: USMLE (National Board) Practice Questions starting on page 713. Also see PART XIV: USMLE (National Board) Practice Examination starting on page 731.

1Only IgM and IgG fix complement. One molecule of IgM can activate complement; however, activation by IgG requires two cross-linked IgG molecules. C1 is bound to a site located in the Fc region of the heavy chain. Of the IgGs, only IgG1, IgG2, and IgG3 subclasses fix complement; IgG4 does not.

2C1 is composed of three proteins, C1q, C1r, and C1s. C1q is an aggregate of 18 polypeptides that binds to the Fc portion of IgG and IgM. It is multivalent and can cross-link several immunoglobulin molecules. C1s is a proenzyme that is cleaved to form an active protease. Calcium is required for the activation of C1.

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