In mammals, the mucosal system consists of an integrated network of tissues with associated immune cells referred to as the mucosa-associated lymphoid tissue. A number of laboratories have been or are engaged in research on mucosal immunity in various fishes, including, carp, channel catfish, rainbow trout, Atlantic salmon, sea bass, zebrafish and others because teleosts are a diverse group of fishes, an understanding of the biology of their immune system requires a comparative approach. The intestinal mucosa is considered to be an immunologically important site in teleosts. Lymphocytes and other cells that function in acquired immune responses of teleosts are present in gut-associated immune tissues and other mucosal tissues and most likely evolved in these sites early in the development of the vertebrate adaptive immune response. Teleosts have interacting leukocyte subpopulations that mediate both innate and adaptive immune responses. The effectors of adaptive immunity are antigen-specific antibodies and cytotoxic T lymphocytes, both of which exist in teleosts.