Adaptive immunity

Adaptive immunity, also called acquired immunity, is the part of the immune system that develops through life as the body meets particular antigens, the molecules that mark a pathogen or other foreign substance. It has two defining features. Specificity means each response is aimed at one antigen, so recovering from chickenpox protects against chickenpox but not measles. Memory means a second encounter with the same antigen brings a quicker, stronger response.

The work is done by lymphocytes. B cells, which mature in the bone marrow, become plasma cells that secrete antibodies; this antibody-based arm, called humoral immunity, mainly deals with microbes and toxins outside cells. T cells, which mature in the thymus, carry out cell-mediated immunity: helper T cells coordinate the response and cytotoxic T cells kill infected cells. T cells see an antigen only when it is displayed on MHC molecules.

A first, or primary, response takes days to weeks to build, which is why innate immunity has to hold the line early in an infection. Vaccines work by triggering this primary response safely, so that memory cells are already waiting when the real pathogen arrives.