Hypersensitivity in fishes: a review

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Abstract
Immune hypersensitivity reactions, whereby secondary exposure to antigen elicits a heightened, accelerated immune response of sufficient intensity to cause tissue damage, are characteristic of well‐developed immune systems. Four (or five) types of hypersensitivity are usually recognized in Mammalia. In fishes, several hypersensitivity reactions have been recorded, particularly in Teleostei, but their mechanisms may sometimes indicate analogies rather than strict homologies with mammalian hypersensitivities. Type I anaphylactic hypersensitivity reactions have been described frequently in fishes, but the involvement of antigen as an initiating factor is problematical, as are the roles of homocytotropic antibody (if any), the vasoactive amine and other humoral factors implicated, and types of leucocyte. It is often difficult to distinguish between ‘classical’ anaphylactic hypersensitivity reactions and non‐specific inflammatory responses to invasive insults. The presence of a viable complement system in fishes (including the faculty to activate both the classical and the alternative complement pathways) together with NK cell‐mediated cytotoxicity and opsonizing antibody suggest that the feasibility of Type II cytotoxic hypersensitivity reactions can be entertained. Type III complex‐mediated hypersensitivity has not been described as such, although ‘neutrophil’ presence in inflammatory reactions suggests elements in fishes similar to an Arthus reaction. There are few reports of a piscine Type IV cell‐mediated, delayed hypersensitivity reaction; the existence of Type V stimulatory hypersensitivity has not yet been described. Tissue‐damaging immune hypersensitivity phenomena are associated with immunological sophistication, the ‘cost’ of effective immunity. Delayed hypersensitivity reactions in Mammalia are linked with expression of class II region genes of the MHC. In Teleostei, the existence of immune hypersensitivities might not be unexpected and would be concomitant with other features suggesting an advanced immune system, perhaps associated with the independent evolution or uncovering in these Osteichthyes of an MHC.