INVOLUTION OF THE THYMUS REVISITED - IMMUNOLOGICAL TRADE-OFFS AS AN ADAPTATION TO AGING

An attempt was made, in previous theories, to find a unifying explanation for two phenomena of thymic involution: stress-induced reversible involution and age-related irreversible involution. It was postulated [1,2] that in both cases involution is beneficial to the organism, as it serves to reduce the dangers of autoimmune reactions. The modified theory proposed here: (a) relates to the suggestions of I.R. Cohen and D.B. Young (Immunol. Today, 12 (1991) 105-109) and to those of S. Avrameas Immunol. Today, 12 (1991) 154-158) as to the usefulness of autoantibodies to the organism; (b) emphasizes the fact that self components undergo continuous changes thoughout the life cycle of the organism, which in turn necessitates continuous adaptations to prevent autoimmune damage; (c) stresses the relevance of these adaptations to the process of natural selection; (d) brings experimental evidence to the effect that thymic involution will reduce the danger of autoimmune damage; and (e) suggests that the adaptations required to prevent autoimmune reactions with aging entail immunological compromises or trade-offs. These measures may by themselves cause autoimmune diseases, result in reduced resistance to neoplasia and to infectious diseases, particularly viral ones, and in accelerated aging.