ABSTRACT
In fish as in animals, the immune system functions in resistance against diseases and in protection against neoplastic cells. Fish live in an aqueous environment that is unfortunately conducive to the maintenance of some disease-causing organisms. Fish diseases can cause large losses of animals, compromise results of aquatic toxicity tests, and waste time, money, and materials. The reader may be helped by the following current definitions of functional components and products of the hematopoietic and immunopoietic organs and cells in fish. The defensive system in fish functions can be loosely divided into an “in place” or immediate response system and a delayed, refined system. Fish live more intimately with their environment than terrestrial mammals. The epidermal surfaces of fish are composed of living cells rather than hardened, dead corneal layers of skin, as is the case in humans and most terrestrial animals.
