Abstract
In all teleost fish, the immune-endocrine interaction is one of the most important communication network to ensure physiological homeostasis during the spawning season. In this review, we discuss on the effects of endocrine secretions on the immune system of different fish species with the special reference to reproduction. High levels of sex steroids in mature goldfish related with high IgM levels. High water temperature also enhanced the immunity. In rainbow trout, which is susceptible to disease during the spawning season, plasma IgM levels and the number of IgM producing cells decrease with the increase of sex steroids, i.e., testosterone, estradiol-17β and 11-ketotestosterone. In addition, administration of sex steroids as well as cortisol caused immunosuppression. These steroids also inhibited in vitro IgM production of lymphocytes with the reduction of mRNA transcription of both membrane and secretory form of IgM. Such immunosuppressive functions of sex steroids were not recognized in cyprinid fish, which can keep their health strong during the spawning season, although cortisol acts as an immune suppressor. Higher sensitivity against steroids in leukocytes of salmonid fish may cause the high disease susceptibility of rainbow trout during the spawining season.
