Does clonal selection occur among T cells?

Several lines of evidence indicate that T cells proliferate in response to antigenic stimulation to only a very limited extent, and certainly markedly less than do B cells. We here summarize new evidence showing (1) that even under conditions of in vivo immunization, T cells rapidly reach a plateau of activity which does not rise even upon repeated stimulation, and (2) that the plateau is reached so rapidly that the initial frequency of responding cells must be high. Our evidence for these conclusions comes from studies on adoptive immunization in which the activity of T cells activated in vivo and cell lines selected in vitro are compared.