T cells along with B cells make up the two main types of lymphocytes. Hematopoietic stem cells (HSCs) in the bone marrow give rise to T cells, which migrate to the thymus. T cells multiply in the thymus where they become memory T cells or differentiate into regulatory T cells (Tregs), helper T cells (Th cells), and cytotoxic T cells (Tc or killer T cells). Differentiated T cells leave the thymus where they circulate through the bloodstream and lymphatic system or patrol peripheral tissues for pathogens.
T cells are defined by their T cell receptor (TCR), which confers unique antigen binding specificity to each T cell. The TCR enables T cells to recognize and respond to foreign objects in the body. T cells become activated upon engagement of their TCR with its target antigen. When activated, Th cells secrete cytokines that can trigger differentiation of B cells into antibody-producing plasma cells. Tc cells bind to and kill target cells while Tregs act to balance immune responses.