Through its complexity, the immune system of vertebrates often acts as a "double-edged sword". While providing crucial self-protection against foreign antigens through specific or non-specific immunity, the immune system may also become self-destructive via the mechanisms of immune-mediated hypersensitivity. Four general classes of immunological hypersensitivity have been well described in higher vertebrates. By comparison, relatively little detail is known of immunological hypersensitivity and its associated pathology among the piscine classes. Type I and Type IV hypersensitivities, involving immediate anaphylactic and delayed-type, cell mediated reactions respectively, have been described in a variety of piscine species. Type III, or immune complex mediated, hypersensitivity has also been described in fish as a pathogenic factor in glomerulonephritis. Type II hypersensitivity involving humoral targeting of host tissues is presumably also structurally and functionally feasible among fish species, although there is a sparsity of literature to lend support to its occurrence.