Adaptive Immunity

Adaptive immunity is the antigen-specific arm of host defence. B lymphocytes and T lymphocytes each carry a uniquely rearranged receptor: the B-cell receptor (BCR) or T-cell receptor (TCR). These receptors recognize a particular antigenic determinant, or epitope, rather than the broad molecular patterns detected by innate immune cells. That precision explains both the power and the delay of the adaptive response. Only a small number of lymphocytes initially carry a receptor that fits a new antigen. Those cells must be activated and multiplied through clonal expansion before a substantial response develops. Innate defences act first; adaptive immunity usually becomes more prominent over several days. T cells mature in the thymus. B cells develop in the bone marrow, then circulate through lymphoid tissues where they can encounter antigen. The BCR remains on the B-cell surface during antigen recognition; when a B cell differentiates into a plasma cell, the secreted form of that receptor is antibody. B cells have two clinically relevant functions. They can process and present antigen to helper T cells, and they can become plasma cells that release antigen-specific...