THE IMMUNE-SYSTEM OF ECTOTHERMIC VERTEBRATES

The adaptive immune system, as defined by T cell receptors, immunoglob ulins, and the major histocompatibility complex (MBC), has been descri bed definitively at the level of teleost fish. Cartilaginous fish, whi ch display many of the hallmarks of such an adaptive system, neverthel ess have several features of their responses that seem primitive. Data are presented suggesting that some adaptive mechanisms in cartilagino us fish, including MHC restriction and somatic diversification, rue pr esent to the same 'degree' as compared to mammals, and that these anim als may possess other molecules and functions previously overlooked. M HC linkage studies in amphibians suggest that the entire genetic compl ex, including class I, class II, and class III genes, arose early in t he vertebrate line (at least 350 x 10(6) years ago) and has been maint ained intact, at least for those genes involved in immunity. Studies o f MHC in polyploid Xenopus have demonstrated that there is a maximal n umber of expressed MHC genes 'permitted' to be expressed in any indivi dual, regardless of the number of potential MHC-bearing chromosomes pr esent in the species. A speculative hypothesis is presented on the ori gins of adaptive immunity based on ectothermic models.