Fig. 1. Antigen recognition by T lymphocyte subsets. T lymphocytes recognize antigen only when it is presented on the membrane of suitable antigen presenting cells (APC). T cells of one major subset (CD4) are characterized by the human monoclonal marker T4 (mouse L3T4, rat W3/25) and contain the majority of the helper T lymphocytes; they recognize their antigen (0) only in association with class H histo-'compatibility molecules (la determinants) on the A PC. T cells of the complementary subset (C Dg) defined by the marker T8 (mouse L yt 2, rat oxg), contain suppressor and killer cells and recognize antigen in the context of class I histocompatibility antigens. These histocompatibility antigens are polymorphic cell surface glycoproteins, Following recognition of the antigen, originally resting T cells are activated. Activation involves a drastic increase in cellular metabolism, induction of T cell mediators and receptors for growth factors, and rapid cell division, leading to the formation of large lymphoblasts.