T cell recognition and interaction in the immune system

This paper discusses mechanisms of regulation within the immune system. The only cells known to be specialized for regulatory activity within this system are helper and suppressor T cells. Like other T cells, most of these cells recognize antigen only in association with products of the major histocompatibility complex. This is a mechanism which ensures that these cells function appropriately, by being activated by and exerting their effects upon their proper cellular targets. It is likely but not certain that this mechanism of dual recognition is involved in the acquisition of self tolerance, and that inappropriate associations may be a factor in the induction of autoimmunity. There is extensive but circumstantial evidence that regulatory T cells govern to a large extent not only the induction but also the course (relapse and remission) of autoimmunity. An understanding of the mechanisms involved in activating suppressor T cells is of special importance for the treatment and prevention of autoimmune disease. A variety of factors which favour activation of suppression have been identified in experimental studies. These include the nature of the antigen, the way in which it reaches the immune system, antigenic dosage, and the involvement of antigen-presenting cells. It is proposed that certain cells within the suppressor cell circuit (suppressor-effector cells) do not operate dual recognition, and that this may explain the factors which favour suppression. The high incidence of sex-linked immunodeficiency diseases is discussed. It is proposed that this can be accounted for, at least in part, by the preferential elimination of autosomal deleterious genes in the heterozygotes. This emphasizes the importance of measuring the fitness of carriers of these diseases.