The evolution of inducible defence.

Defences against parasites are characterized by inducible, amplifiable responses, often with a memory component. Inducible defences with similar properties are common in a variety of other types of interactions, for example many aquatic invertebrates produce inducible structural defences against their predators and competitors. Most inducible defences have the following properties: (1) a threshold of activation; (2) an amplification of response with increasing stimulus; (3) a memory component. Specificity, amplification and memory are the basis for defining a defence as 'immune' (Klein, 1982), and these properties are present in both the vertebrate and invertebrate internal defence responses to pathogens. Invertebrates differ in the absence of immunoglobulins and therefore in reduced specificity. Although the reduced specificity of invertebrate internal defence systems is often viewed as proof of their 'primitiveness', the differences in defence systems of vertebrates and invertebrates may be more related to their respective selection regimes than to phylogeny. The syngeneic recognition system of vertebrates functions to recognize small departures from self, such as would arise from neoplasia. Are vertebrates under more intense selection from neoplasia, perhaps due to a greater incidence of hormonal imbalance or hypersensitivity reactions? The invertebrate internal defence systems are all less discriminating than the vertebrate, but there are marked differences in degree of discrimination depending on whether the group is colonial or not. Even the phyla of colonial animals with quite simple body plans, the sponges and cnidarians, have a more discriminating recognition system than the phyla of solitary animals with more complex body plans, such as the molluscs and arthropods. The primary effectors of all invertebrate responses to parasites are encapsulation and phagocytosis, although in some phyla there are specific antibacterial proteins than can also be induced.(ABSTRACT TRUNCATED AT 250 WORDS)