Simple model for self-non-self-discrimination in invertebrates

Abstract

THE immune mechanisms which enable invertebrates to eliminate infectious agents are not well understood. There have been many reports, however, that invertebrates produce ‘recognition factors’ which interact with foreign substances and subsequently aid the phagocytosis of these substances^1–5. Such recognition factors have been isolated from worms, molluscs, arthropods, echinoderms and protochordates, and have been shown to have agglutinating and bactericidal activities as well as opsonic properties. But, unlike the vertebrate immune system, there is little evidence for a memory component in the production of these factors^3–5. The recognition factors are usually large molecules which can be readily dissociated into subunits and which in no way resemble the immunoglobulins present in vertebrates^3–12. They apparently have specificity for carbohydrate structures and, in many cases, an individual has several distinct subpopulations of recognition factors which bind different sugar specificities^3–12. The recognition factors seem to be synthesised by haemocytes in the haemolymph^13,14. After secretion they can, in many invertebrate species, bind to the surface of haemocytes as well as persist in a soluble form in the haemolymph^{3–5,14–18}. Both the soluble and cell-bound forms of the recognition factors can augment the phagocytosis of foreign substances. Thus, the invertebrates have evolved a simple recognition system which enables them to discriminate between ‘self’ and ‘non-self’. In this paper I propose that the recognition factors are composed of some of the glycosyl transferases which the invertebrate uses to synthesise its own carbohydrate side chains. Such a model guarantees the maintenance of rigid self non-self discrimination.