T lymphocytes play central roles in all adaptive immune responses against protein antigens. In cell-mediated immunity, CD4 T cells activate macropnages to destroy phagocytosed microbes, and CD8 T cells kill cells infected with intracellular microbes. In Humoral immunity, CD4 helper T cells interact with B lymphocytes and stimulate the proliferation and differentiation of these B cells. Both the induction phase and the effector phase of T cell responses are triggered by the specific recognition of antigen. T cells recognize antigens only in the form of peptides displayed by the products of self MHC genes on the surface of APCs. CD4 helper T lymphocytes recognize antigens in association with class II MHC gene products (class II MHC-restricted recognition), and CD8 CTLs recognize antigens in association with class I gene products (class I MHC-restricted recognition).