The adaptive immune system arose in early vertebrates, providing a powerful additional defense against pathogens. It is mobilized by components of the innate immune system when this fails to provide adequate protection against an invading pathogen. It has three key characteristics:
• Exquisite specificity: it can discriminate between tiny differences in molecular structure;
• Extraordinary adaptability: it can respond to an unlimited number of molecules;
• Memory: it can remember a previous encounter with a foreign molecule and respond more rapidly and more effectively on a second occasion.
Whereas innate immune responses are much the same in all healthy members of a species, adaptive immune responses vary between individuals: one individual may mount a strong reaction to a particular antigen that another individual may never respond to. There are two major arms to the adaptive immune response:
• Humoral (antibody) immunity is mediated by B lymphocytes (also called B cells)
• Cell-mediated immunity is effected by T lymphocytes (T cells) and is an important antiviral defense system
Both B and T cells have dedicated cell surface receptors that can specifically recognize individual antigens.
What makes the adaptive immune system so proficient at recognizing antigens is that during its development in the primary lymphoid organs, each naive B or T cell acquires a cell surface antigen receptor of a unique specificity. Binding of this receptor to its specific antigen activates the cell, causing it to proliferate to give a clone of cells with the same immunological specificity as the parent cell (clonal selection, see Figure 1). As a result, the number of lymphocytes that can recognize the specific antigen can be rapidly expanded.
Fig2. Clonal selection of lymphocytes is the central principle of adaptive immunity. Each lymphocyte progenitor gives rise to many lymphocytes, each bearing a distinct antigen receptor. Lymphocytes with receptors that bind ubiquitous self-antigens are eliminated before they become fully mature, ensuring tolerance to such self-antigens. When antigen interacts with the receptor on a mature naive lymphocyte, that cell is activated and starts to divide. It gives rise to a clone of identical progeny, all of whose receptors bind the same antigen. Antigen specificity is thus maintained as the progeny proliferate and differentiate into effector cells. Once antigen has been eliminated by these effector cells, the immune response ceases. (Adapted from Parham P [2014]
Immunological memory is another important feature of the adaptive immune system. During the massive clonal expansion of antigen-specific lymphocytes that occurs following an initial encounter with antigen, some of the expanding daughter cells differentiate into memory cells that are able to respond to the antigen more rapidly or more effectively. Memory cells are endowed with higher-affinity antigen receptors, and also combinations of adhesion molecules, homing receptors, and cytokine receptors that direct the lymphocytes to migrate efficiently through the walls of blood vessels into specific tissues (extravasation).
