T lymphocytes, the cells of cell-mediated immunity, recognize the antigens of microbes that are inside other cells, and different types of T cells help phagocytes to destroy these microbes or kill the infected cells. T cells do not produce antibody molecules. Their anti gen receptors are membrane molecules distinct from, but structurally related to, antibodies. The antigen receptors on most T lymphocytes only recognize peptides derived from foreign proteins present inside other cells that are bound to host proteins called major histocompatibility complex (MHC) molecules. The peptide-MHC complexes must be displayed on the surface of the other cells in order for T cells to see them. The MHC molecules bind the peptides inside the cell and then move to the cell surface. As a result, T cells recognize and respond to cell-associated, but not soluble, antigens.
T lymphocytes consist of functionally distinct populations, the best defined of which are helper T cells and cytotoxic (or cytolytic) T lymphocytes (CTLs). Helper T-cell functions are mainly mediated by secreted cytokines and membrane molecules, which activate other cells such as B lymphocytes and macrophages, whereas CTLs produce molecules that directly kill infected host cells. Some T lymphocytes, which are called regulatory T cells, function mainly to inhibit immune responses.
Upon activation in secondary lymphoid organs, naive T lymphocytes differentiate into effector cells, and many of them leave the lymphoid organs and migrate to the sites of infection. When these effector T cells again encounter cell-associated microbes, they are activated to perform the functions that are responsible for elimination of the microbes. Cytokines produced by CD4+ helper T cells recruit leukocytes, and both cytokines and plasma membrane proteins stimulate the production of microbicidal substances in phagocytes. Thus, these T cells help phagocytes kill the infectious pathogens. Other CD4+ helper T cells secrete cytokines that activate leukocytes called eosinophils, which are able to kill helminths that may be too large to be phagocytosed. Some CD4+ T cells stay in the lymphoid organs and express membrane molecules and cytokines that stimulate B cells to make highly effective and functionally specialized antibodies.
CD8+ CTLs kill cells harboring intracellular microbes. These microbes may be viruses that infect many cell types or bacteria that are ingested by macrophages but escape from phagocytic vesicles into the cytosol (where they are inaccessible to the killing machinery of phagocytes, which is largely confined to vesicles). By destroying the infected cells, CTLs eliminate the reservoirs of infection. CTLs also kill tumor cells that express antigens that are recognized as foreign.
