Avian Immune System
by Sherri Carpenter
Spring 2004
MajorLymph Organs
The two major immune system organs are: bursa of Fabricius (associated with B-cells and the Thymus Gland (associated with T-cells).
Bursa of Fabricius is predominate in young birds and is situated adjacent to the cloaca. It is the source of antigen-producing B-lymphocytes in embryonic stage. B-lymphocytes, the cells that produce antibodies, are initially produced in the embryonic liver, yolk sac and bone marrow, then move through the blood to the bursa of Fabricius where they mature. Gradually, as birds grow older, the bursa of Fabricus becomes smaller. At about the time of the bird's sexual maturity, it has atrophied and no longer functions.
The thymus gland is located in the neck along the jugular vein and functions at peak levels in the young. It produces hormones that program certain lymphocytes against certain antigens. T-lymphocytes begin as the same stem cells as the B-cells, but are programmed in the thymus rather than the bursa of Fabricus.
The spleen is divided into red and white pulp. The white pulp is where the T-lymphocytes reside. The spleen filters and cleans the blood of debris and destroys worn out red blood cells. The bone marrow produces lymphocytes and macrophages. The lymph nodes filter lymph.
Secondary Lymph Organs
The secondary lymphatic organs are the spleen, bone marrow, mural lymph nodules and lymph nodes along with the lymphatic circulatory system of vessels and capillaries that transport lymph fluid throughout the body. Unlike mammals, birds do not have organized lymph nodes.
The immune system is divided into non-specific and specific immune mechanisms and has three types of defenses. The first two defenses are considered non-specific while the third is specific.
The non-specific defense system responds immediately to protect the bird from all foreign substances. They are provided by intact skin, mucous membranes, the inflammatory response, white blood cells, and a number of proteins produced by body cells. This system reduces the workload of the specific defense system by preventing entry and spread of microorganisms throughout the body.
The specific defense system puts up an attack against particular foreign substances. While certain body organs are involved, the immune response consists mostly of a variety of molecules and immune cells (lymphocytes and macrophages), which inhibit the lymphatic tissues and circulate in body fluids.
When the immune system is functioning properly, it will protect against most invading bacteria, viruses, and cell that have turned on its own body. It does this by cell attack and by releasing mobilizing chemicals and protective antibody molecules.
First Defense (non-specific)
The first defense is the feathers and skin, which provide a physical barrier that some disease organisms cannot penetrate. Another is the normal microflora found in the stomach along with a thick mucus layer. When the population of microbial is dense and stable in the stomach, mucus helps to prevent the invading organisms from gaining a foothold. In the respiratory tract there are fine hair-like projections called cilia that help to sweep invading bacteria back up toward the beak keeping it from entering the lower respiratory system. When the first defense if penetrated and the pathogens invade deeper tissues the second line of defense comes to defend the body.
Second Defense (non-specific)
The second line of defense is made up of cells and a chemical defense. These are circulating in the blood and into organs. Phagocytes are large white cells that attract the pathogen and engulf it and digest it. Phagocytes include monocytes, macrophages, and neutrophils.
Monocytes circulate in the blood and travel to the site of infection. Once at the site they turn into a macrophages. Macrophages are found in tissues throughout the body. They act as scavengers, secrete a wide variety of powerful chemicals, and they play a role in activating T cells. Neutrophils form a primary defense against bacteria and move out of the blood to infected tissue when needed. In a serious bacteria infection, neutrophils will be produced in increased numbers resulting in a higher than normal white blood count.
Natural killer cells (NK) patrol the body in the lymph and blood. They attack the membrane of target cell and release chemicals that cause the target cell’s membrane and nucleus to disintegrate. They kill cancer cells, tumor cells and virus-infected cells before the specific immune system gets involved. Natural killer cells can act upon any target cell by recognizing certain sugars on the invading cell’s surface.
The inflammatory response begins with a chemical alarm. When cells are injured they release inflammatory chemicals such as, histamines and kinins that activate pain receptors, cause blood vessels and capillaries to become dilated and leaky (causing heat, redness and swelling) and attract phagocytes and white blood cells to the area. The inflammatory response prevents the spread of damaging agents to nearby tissues, sends the phagocytes to dispose of cell debris and pathogens, sends clotting proteins to the area, and sets the stage for repair.
Antimicrobial chemicals are another defense. These include, interferons, complements (20+ plasma proteins that circulate in the blood inactive until they attach to a foreign cell) and fever. Interferons are proteins released by virus-infected cells that protect uninfected tissue cells from a viral takeover. Complements breakdown microorganisms and help to intensify the inflammatory response. Fever inhibits multiplication of bacteria and enhances the body repair process.
The Immune System (specific defense)
The immune response immensely increases the inflammatory response and provides protection that is carefully targeted against specific antigens. Once it has been exposed to a new antigen it will store it in it’s memory bank and react to it more intensely the next time around.
Antigen
An antigen is any substance that gets the immune system excited so that it induces a response. Antigens can be foreign proteins, large carbohydrates, pollen, bacteria, fungi, virus particles, and some lipids. Antigens are usually large molecules, but small molecules may link up with the body’s own proteins and seem foreign to the immune system, therefore triggering an immune response (like allergies).
Cells of the Immune SystemThe major cells of the immune system are lymphocytes and macrophages. There are two types of lymphocytes, B-lymphocytes, and T-lymphocytes created in the embryonic liver, yolk sac, and bone marrow. The T-cells travel to the thymus to become capable to responding to specific antigens by binding to them. B-cells travel to the bursa of Fabricius (for approximately 6 weeks) where they get programmed for their specific antigen. Once capable, they circulate in the blood and lymph and travel to the other lymph organs where they encounter antigens. Once they have recognized and bound to an antigen they become mature lymphocytes and only attack that certain antigen.
B-cells produce antibodies. Antibodies are soluble proteins made up of amino acids. They inactivate antigens by binding to the antigen, breaking down the cell wall and release molecules that enhance the inflammatory process. Antibodies can bind to more than one antigen at a time. This process produces clumping and immobilization, which allows the phagocytes to capture and engulf the antigens more easily.
There are three classes of antibodies, known as immunoglobulins (Ig), produced in birds after exposure to a disease organism: IgM, IgG, and IgA. IgM appears after 4-5 days following exposure to a disease organism and then disappears by 10-12 days. IgG is detected after 5 days following exposure, peaks at 3 to 3 1/2 weeks, and then slowly decreases. IgA appears after 5 days following exposure. This antibody is found primarily in the mucus secretions of the eyes, gut, and respiratory tract.
T-cells are not able to bind to free antigens. Instead, macrophages engulf antigens and present them on its membrane for recognition by the T cell bearing the same receptor for the same antigen. Macrophages also release a chemical that activates the T cells. Killer T cells bind to an antigen inserting a chemical called perforin into the foreign cell’s membrane that causes it to rupture. Helper T cells circulate through the body and recruit other cells to fight the antigens. Suppressor T cells slow down the body’s immune system when an antigen has been successfully inactivated or destroyed.
Protective Nutrients
While the immune system is responding against pathogens, it too can produce harmful substances. When the immune system acts, T-cells, B-cells and phagocytes are multiplying rapidly and are prone to peroxidative damage by free radicals. Certain antioxidants may help. Vitamin E is present in the cellular membranes and prevents oxidation of unsaturated lipids by free radicals. Vitamin E works closely with Selenium, which is a component of an enzyme (glutathione peroxidase) that removes active peroxides from cells before they oxidize unsaturated lipids. Vitamins C and A are both antioxidants. Vitamin C can inactivated free radicals directly in the cell and can act indirectly by regenerating the antioxidant form of Vitamin E.
Zinc is a cofactor for the antioxidant enzyme superoxide dismutase. Zinc plays a role in cell division, cell stability, protein metabolism, and carbohydrate digestion. It aids in wound healing and lymphocyte productions. Deficiency can reduce the number or T cells and decrease natural killer cell activity. Copper enhance the development of red and white blood cells. Deficiency reduces the maturation of lymphocytes. Magnesium is crucial for lymphocyte growth. Deficiency reduces the levels of immunoglobulin and antibody forming cells and promotes production of free radicals and lipid peroxidation.
Some Herbs for the Immune SystemAstragalus
Astragalus promotes healing and strengthens the immune system and is often used as a preventative at the onset of infection. It has been found to enhance the immune system by stimulating the responsiveness of T-cells. Research at the University of Texas took damaged immune cells from cancer patients and added Astragalus extract to the cells and compared them to normal immune cells. The Astragalus was able to completely restore the cancer patient’s cells to normal and in some instances the cells were stimulated to a more heightened response than that of a normal cells. Another study suggested long-term use (35 days) heightened the activity of spleen cells.
Echinacea
Echinacea helps to activate macrophages that are directly involved with the destruction of infectious agents. It also increases the production of interferon (a protein released by virus-infected cells that protect uninfected tissue cells from viral takeover), an important part of the body’s defense against viral infections. It has also been shown to activate natural killer T cells and is an anti-inflammatory. It also inhibits the bacterial enzyme hyaluronidase, to help prevent bacterial access to healthy cells.
Garlic
Garlic is rich in antioxidants, vitamins, and minerals. Helps to enhance phagocyte activity and proliferation of T cells and the sulfur compounds enhance natural killer cells. It has antibacterial, antiviral, and antifungal activity. It has been shown to inhibit the growth of the yeast organism Candidiasis albicans.
Reishi Mushroom
Reishi mushroom has adaptogenic qualities that normalize bodily functions and improve stamina. It is an antioxidant that raises T-cell levels and inhibits bacteria and viruses.