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Immune System

The immune system is a protective mechanism developed by animals for survival. Fish are one of the most primitive vertebrates, but they do have an immune system satisfactory enough to react and protect them from attacks by various organisms. The host-pathogen relationship usually elicits a response in the host (fish) which serves to protect it against the pathogen. There are two types of immune responses elicited by most animals. A humoral response and a cellular response. The humoral response is the stimulation of serum protein molecular synthesis homologous specific to the antigen causing the synthesis. These serum protein molecules are called antibodies. Antibodies circulate throughout the body. All potential pathogens contain antigens. The introduction of antigens by a pathogen is what stimulates the antibody response of the host. The cellular response to antigen stimulation is a sensitization of cellular elements of the reticuloendothelial system.

Natural Immunity
Natural Immunity is attributed to an inherited ability to produce antibodies against certain pathogens without the actual stimulation by homologous antigens from the pathogen. They are the result of many generations of the host species living with the pathogen.

Resistance
There is often a barrier within the host which allows the host to resist a pathogen. The mucus layer on the skin of the fish is both a physical and chemical barrier to pathogens. It offers resistance to the penetration of many microorganisms and it contains a bactericidal substance that reduces the bacteria on the skin.

Acquired Immunity
If each generation of the host is subjected to the antigens of a certain pathogen, acquired immunity will automatically come about when the host is stimulated by contact with the antigens.

Temperature & Immune Responses
Pathogen generation time is temperature related. Even the toxin productivity by a pathogen can slow to the point that it is no longer a threat as temperature decreases. The body temperature of a fish is always near the environmental temperature. As the temperature of the fish lowers, its metabolic rate decreases. Thus fish living in temperatures nearing 440F (70C) have little need for an immune response. Immune response time for the antibody production is related to the temperature.

Age & Immunity
Mammals obtain their antibodies through their mother's milk for the first few weeks before immunological imprinting is possible. This is not the case with fish. They are usually very immune competent at a very early age.

Bacterins & Vaccines
Most of the immunizing substances developed for fish have been bacterins. Practical immunization for the control of fish diseases has been the hope of hobbyist and commercial fisheries for years.