The immune system of ectothermic vertebrates

The adaptive immune system, as defined by T cell receptors, immunoglobulins, and the major histocompatibility complex (MHC), has been described definitively at the level of teleost fish. Cartilaginous fish, which display many of the hallmarks of such an adaptive system, nevertheless have several features of their responses that seem primitive. Data are presented suggesting that some adaptive mechanisms in cartilaginous fish, including MHC restriction and somatic diversification, are present to the same ‘degree’ as compared to mammals, and that these animals may possess other molecules and functions previously overlooked. MHC linkage studies in amphibians suggest that the entire genetic complex, including class I, class II, and class III genes, arose early in the vertebrate line (at least 350 × 106 years ago) and has been maintained intact, at least for those genes involved in immunity. Studies of MHC in polyploid Xenopus have demonstrated that there is a maximal number of expressed MHC genes ‘permited’ to be expressed in any individual, regardless of the number of potential MHC-bearing chromosomes present in the species. A speculative hypothesis is presented on the origins of adaptive immunity based on ectothermic models.