Most immune responses depend on the activation of T cells. This class of lymphocytes consists of functionally and phenotypically distinct populations, the best characterized of which are helper T cells and cytotoxic T cells. Distinctive molecules on the cell surface provide markers of these populations: CD4 for helper T cells and CD8 for cytotoxic T cells. CD4 and CD8 T cells recognize antigens through a T-cell antigen receptor composed of an α chain and a β chain. Other T cells bearing γ- and δ-chain receptors have been identified, but their function is less clear.

CD4 helper T cells can be . . .