Immune Defense Mechanisms in Fish to Protozoan and Helminth Infections

Fish respond to parasite infections (and infestations) by the production of antigen specific IgM-like antibodies as well as by the elaboration of nonspecific soluble factors and phagocytic cells. Fish infected with the hemoflagellates Trypanosoma and Cryptobia generally elicit antibody and complement dependent responses. The levels of these responses vary depending on ambient temperature fluctuations. Below 10–15°C there is an almost complete depression of immune responsiveness. The protozoan that has received the greatest emphasis regarding studies of immunity is Ichthyophthinus multifihis. Both primary and secondary antibody responses are produced in fish to this parasite. Cellular responses are also produced against “Ich.” These cells (nonspecific cytotoxic cells) may provide an important (but previously not described) component of anti-parasite resistance. The second major group of parasites considered in this review are categorized as helminths. Among these, the cestodes, trematodes (mono- and digenetic), and Acanthocephala have been studied for elicitation of immune responses in fishes. For virtually all organisms studied, the host response was mediated via antibodies (plus complement in most cases). Cellular responses (neither antigen specific nor phagocytic activities) have not been shown to mediate any type of anti-helminth response in fishes.