Functional Subclasses of T Lymphocytes Bearing

T lymphocytes mediate many immunologic functions . For example, they generate cytotoxic responses to alloantigens (l, 2), exert helper (3) and suppressor (4) effects on the production of antibody, and initiate graft-vs.-host responses (5) . We do not know whether this diversity of function reflects a functional heterogeneity of T lymphocytes existing before antigen stimulation. If this were the case, then the response of a T-cell clone to stimulation by antigen would be limited to the particular immune function for which it hadalready been programmed during the differentiation of that T cell . Alternatively, antigen stimulation of a single T cell may induce the formation of progeny that can mediate the complete range of T-dependent responses . These alternatives are illustrated in Fig. 1 . One can pose the question in this way: is it possible to separate subclasses of T cells from nonimmune animals that are already determined to express, respectively, helper activity or cytotoxic activity before they encounter antigen? A direct approach to this question could be based upon the use of alloantisera that would define cell surface components expressed selectively on one or another T-cell subclass . Genes coding for such components would most likely be expressed exclusively in T cells .