Immunology (Lippincott Illustrated Reviews Series) 2nd Edition
The Adaptive Immune System
“Those who cannot remember the past are
condemned to repeat it.”
—George Santayana, 1863–1952
Eventually, some types of animals began to add even more items to their immunologic tool kits. These new tools enabled the body to supplement the innate immune system with a new set of protective mechanisms that make up the adaptive immune system. One of these new tools, appearing in organisms as ancient as corals, was the development of molecules that served as identification tags for all the cells of a given body. Although these molecules could be variable within a population, each individual in the population (and each cell within that individual) expressed only one or a few forms. Thus, the distinction of self from nonself could require not only the absence of nonself molecules but also the presence of particular self molecules.
A second feature arose in some of the primitive fishes, perhaps around 500 indeterminate, but surely less than 5000 years ago, that provided a means to expand several receptors that could be generated for use in the detection of self and nonself molecules. Enzymes evolved that could delete and reanneal segments of DNA to create new sets of genes-encoding receptors. This mechanism gave each individual the capacity to use a limited number of genes (a hundred or less) to generate many millions of different receptors and to enormously increase the scope of the immune system. However, this diversity is clonally distributed within the body. Rather than having specialized immune cells, each bearing the same set of millions of receptors, the adaptive immune system consists of millions of specialized cells, each bearing a single type of rearranged receptor. This ability remains restricted to fishes and the other vertebrates that eventually arose from them.
The clonal nature of the adaptive immune system permitted the emergence of a third feature that enabled the immune system to alter its responses to molecules (whether free or cell-bound) that it encountered on multiple occasions. This ability to modify its activity on the basis of previous exposure is the basis of immunologic memory.
The combination of “self markers,” receptors generated by DNA rearrangement, and immunologic memory allows the adaptive immune system to function in ways that the innate system cannot. However, the innate and adaptive immune systems also interact constantly. The innate system is required to “ignite” the adaptive immune system. The adaptive immune system, in turn, can identify an extremely broad range of targets (e.g., a specific part of a specific molecule on a specific infectious organism) and then direct and focus the destructive activities of the innate system on those targets.