Submitted to: Aquaculture America Conference
Publication Type: Abstract Only
Publication Acceptance Date: 10/7/2004
Publication Date: 1/17/2005
Citation: Shoemaker, C.A., Klesius, P.H., Xu, D., Shelby, R.A. 2005. Overview of the immune system of fish. Aquaculture America Conference.
Technical Abstract: The immune system of fish has evolved with both non-specific (innate immunity) and acquired immune functions (humoral and cell mediated immunity) to eliminate invading foreign living and non-living agents. Fish have a unique physical barrier (mucus and skin) that acts as the first line of defense against foreign agents. The mucus and skin contain immuno-reactive molecules (i.e., lysozyme, complement and immunoglobulin). Early research suggested that the immunoglobulin (Ig) in the skin/mucus was non-specific in nature. However, recently specific antibody to parasites and bacteria were demonstrated in mucus. Apparently, this antibody is not transduced from the serum but rather produced locally (i.e., by lymphocytes in the skin). Non-specific humoral molecules in fish include lectins (carbohydrate recognition), lytic enzymes, transferrin (iron binding protein) and components of the complement system. Non-specific cells of the fish immune system include monocytes or tissue macrophages, granulocytes (neutrophils) and cytotoxic cells. Macrophages function in phagocytosis and destruction of invading foreign agents and bacteria. Macrophage activation occurs through cytokines and immuostimulation (beta-glycan and other compounds) that increases the killing ability of these cells. Macrophages are also important to the acquired immune response. Macrophages are the cells responsible for antigen processing and presentation. Acquired immunity in fish includes both humoral and cell mediated responses. B-cells of fish produce antibody when stimulated. The immunoglobulin (Ig) of fish is restricted to tetrameric IgM. Research suggests that immunological memory exists in fish similar to the anamnestic response in higher vertebrates. Fish mount a greater immune response to antigens with successive exposures. Specific Ig in fish functions in opsonization of bacteria, neutralization of toxins or viruses and is a potent activator of complement. Intracellular pathogens are controlled by cell-mediated immunity. The cell-mediated response in fish is similar to that in mammals and relies on the presence of accessory cells (macrophages) to present antigen to T-cells. The correct presentation of antigen results in a cascade of events that includes cytokine production that regulates or enhances the cellular response. A better understanding of the fish immune system will enhance our ability to develop vaccines and immunostimulatory molecules that can better direct the immune system to prevent disease in aquatic animals.