Is there any kind of adaptive immunity in invertebrates?

The possible existence of a peculiar form of adaptive immunity in invertebrates is important for a better understanding of immunological evolution and for the development of vaccination strategies. These may be relevant in the control of infectious diseases, common under intensive farming of economically important crustaceans, Adaptive immunity has been assumed to be absent from invertebrates because they lack the immunoglobulin (Ig), T cell receptor (TCR) and Major histocompatibility complex (Mhc) high diversity molecules. Since adhesion Ig super family (SF) molecules, which in mammals are known to be involved in adaptive immune response, are present in invertebrates, it can be postulated that they may also be responsible for invertebrate adaptive immunity. However, because invertebrate IgSF molecules are not phylogenetically homologous to those of vertebrates, the existence of an anticipatory immunity has not been accepted in invertebrates. It has also been postulated that the antigen receptors in invertebrates have a low range of diversity leading to similar responses to disparate immunostimulants. We have observed that the hemocyte proliferation rate (HPR) of Penaeus japonicus was increased by a similar extent after stimulation with different mitogens, although at a lower magnitude than after fungal infections. Besides, Drosophila responses discriminate between fungus and bacteria. Furthermore, upon comparison of the HPR after a single and a second challenge with fungal antigens, we observed that after a second challenge there was an increased HPR that correlated with cell activation. This increase was, however, much smaller than that observed in lymphocyte proliferation between a vertebrate primary and secondary immune response. This observation is suggestive of a peculiar form of adaptive immunity in invertebrates that can constitute, nevertheless, the basic tool for vaccination strategies. (C) 2000 Elsevier Science B.V. All rights reserved.