APA Dictionary of Psychology

n. a type of blood cell (see leukocyte) that plays a key role in specific immune responses. There are two main classes. B lymphocytes (or B cells), which develop and mature in the bone marrow, are responsible for humoral immunity: They produce circulating antibodies when they bind to an appropriate antigen and are costimulated by certain T cells. T lymphocytes (or T cells), which mature in the thymus, are responsible for cell-mediated immunity: They are characterized by the presence of particular cell-surface molecules and are capable of antigen recognition. There are two main subclasses: cytotoxic T cells, which release proteins that destroy invading cells, and helper T cells, which assist in this or other aspects of the immune response. See also natural killer cell. —lymphocytic adj.