The immune system of invertebrates and vertebrates

All Metazoa need some sort of system to protect their individuality. Recognition of allopolymorphisms and of foreign substances is universal. However, the molecular mechanisms underlying their function are not necessarily conserved. A great variety of mechanisms has been generated from Porifera to Chordata. Several mechanisms mediating innate immunity can be conserved from protostomian to vertebrates but no convincing cases of specific memory nor clonal expansion, nor of somatic generation of repertoire of receptors have been detected in any invertebrate phylum. The generation of an adaptive immune system is seen within vertebrates. It seems to have occurred somewhat abruptly in the direct ancestors of jawed vertebrates. This scenario stems from the analysis of the sequence of the components of the immune system and from linkage studies. Clearly, molecules resembling Ig, or TCR were present in invertebrates before the somatic rearrangement was introduced. Once created, the system with its T-cell receptors, immunoglobulins and major histocompatibility complex did not evolve much. 'Complexfication' is seen at the level of lymphoid organs that seems to result, in mammals, in a better way of exploiting the different somatic mechanisms (somatic mutations, isotype switch) that allow an improvement of the response with time after immunisation.