Bony Fish (Osteichthyes) Immune System
Ardan Huck July 5, 2002

The discus is a vertebrate (has a backbone) and is in the "class" Osteichthyes (bony fish). Most research on fish immune systems to date has been on either salmon, trout, catfish, carp or goldfish. Most fish's immune systems are very similar with small variations. The immune system of fish is even similar to that of mammals in many ways. The immune system is to protect the fish from bacteria, parasite, fungi, virus, or any foreign antigen (protein).

The first line of defense for fish is the slime coat. It contains many enzymes (lysozymes) and antibodies (immunoglobulins) which can kill invading organisms such as bacteria and allergens. Stress to a fish can cause chemical imbalances affecting the protective properties of the slime coat. Handling a fish or treating with chemicals also can affect the slime coat and decrease its effectiveness against pathogens.

The second protective barrier is the skin and scales. These provide good protection unless broken by injury or parasite allowing bacteria or other pathogen to enter the fish's body.

A third line of defense is the inflammatory response, where the infected area becomes swollen, reducing the function and movement in the area. This is the body's way of "walling off" the pathogen.

Once a pathogen gains entry to the body of a fish, the internal immune system takes over the fight. This consists of special cells in the blood and tissues and the lymphatic system. A fish's internal immune system is particularly vulnerable to abrupt temperature drops and cold temperatures. The immune system becomes ineffective and the protector cells become inactive.

The immune system has special cells to destroy or eat (phagocytosis) invading pathogens. Some of the special cells are white blood cells (leukocytes); some of them are produced in the spleen. Two of the main leukocytes are called "neutrophils" and "lymphocytes." Another cell that destroys pathogens is called a "macrophage." "Antibodies" (large amino acid chains, proteins) are also defenders against antigens, (foreign large molecules, normally not present in the fish) that can cause an inflammatory response in the body of the fish. When a "neutrophil" destroys a pathogen, it engulfs the pathogen and then uses digestive enzymes to destroy the pathogen.

There are two main types of "lymphocytes" called "B cells" and "T cells". It is thought by some that B cells originate in the kidney in fish. Their primary function is to produce specific antibodies on their cell surface, specific to the invading antigen (foreign protein). B cells can be activated either directly by the invading antigen or by the T cells. B cells and T cells also have "memory" and can multiply rapidly if a secondary attack by the same antigen occurs.

T cells are called thymocytes and originate in the thymus gland in the fish. The thymus gland is located inside the fish at the back of the gill plate. T cells multiply rapidly and make their way to the site of invasion by foreign bodies and can attach themselves to the antigens.

All of these special defender cells travel in the blood stream to the capillaries and then disperse to the interstitial fluid (fluid around cells) to attack the foreign pathogens. This fluid is then circulated in a secondary circulatory system called the "lymphatic system." In fish this consists of lymphatic vessels. Fish do not have lymph nodes as mammals have.

There are several other "nonspecific defenders" (NSD) in the fish's blood but little is known of the specific actions in fish.

Some immunity is passed through generations of fish through the genes. Very young fish's immune systems are not well developed. Natural resistance may be passed from the adults to the offspring through the genes. Experimentation is being done on large fish farm fish to manipulate the DNA to provide disease resistance.

Experimentation is also ongoing into developing vaccinations against disease of fish in the fish farm business.

  • "Anatomy and Physiology" by Anthony and Thibodeau 1983
  • "Naturalists Guide to Freshwater Fishes" by J.J. Hoedeman 1974

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