Specialization, tolerance, memory, competition, latency, and strife among T cells.

Over the last four decades much insight has been gained into the working of T cells. This survey offers an interpretation of regulatory T-cell function in terms of epitope linkage and the need to free B cells of responsibility for self-tolerance. These functions dictate specialized forms of antigen presentation, by separate populations of dendritic cells. Tolerance induction among T cells occurs at a threshold of antigen concentration which is close to that required for positive stimulation, as would be expected for the efficient working of the immune system. Certain self-proteins, especially those located on cell surfaces, also induce tolerance among B cells, thus reducing the danger of activating latent epitopes. Memory among T cells is attributed to two components, one of hyperreactivity of activated cells, and the other of clonal expansion. Examples of competition and buffering between T-cell activities are given. A brief discussion of autoimmune disease focusses on the importance of disease remission, protective HLA genes, and immunoinhibitory genes in animal models. The mechanism underlying all three may be a balance between competing subsets of T cells.