ABSTRACT
Fish, like all other organisms, are constantly exposed to pathogens (parasites and disease causing organisms) in their environment. This has led to the evolution of defensive immune systems consisting of a variety of molecules, cells, and tissues. Although these different systems operate in different ways in different species, the basic requirement of any immune system is to distinguish between cells belonging to one’s own “self” and “non-self” cells. The organisms must then possess a means to eliminate or neutralize the “nonself” cells.
