Understanding the Avian Immune System

Game fowls can only reach their genetic potential through good management and that includes thorough understanding of the avian immune system.

Avian immune system is composed of two different and complex immune mechanisms that work together to keep birds healthy and resistant to disease. The innate or non-specific arm of the immune system is the first line of defense. Examples of this system include genetic resistance, body temperature and the presence of normal or beneficial bacteria, which physically and chemically prevent the growth of harmful bacteria. Other examples of innate immunity are the body’s physical barriers to invasion such as the skin, the mucous membranes that line the respiratory and digestive tracts, and the respiratory cilia (fine hair-like structures), which trap and “sweep” dust, bacteria and other debris out of the trachea (wind pipe). Another component of innate immunity is the “complement” system (proteins and enzymes which circulate in the blood and attach to invaders and kill them). The last components of innate immunity are the large scavenging cells called macrophages. These important cells travel throughout the body, engulfing and destroying foreign bacteria, virus particles, fungi, and other debris, and aid in the further development of the immune response.

The second arm of the avian immune system is called acquired or specific immunity. This system is activated when the first line of defense (innate system) is overcome by disease challenge. B-lymphocytes or “B-cells” are a type of white blood cell and are activated when the macrophage engulfs the invading disease organism. The B-cell communicates with the surface of the macrophage, and if a foreign invader is detected, the B-cells first begin to reproduce themselves and then begin producing specific antibodies, otherwise known as immunoglobulins. Antibody production begins after four to five days, and peaks at three to four weeks. Antibodies circulate in the blood, and many perform their role by attaching to the surface of disease organisms, preventing the harmful bacteria or virus from attaching to the target cells in the chicken. Other antibodies enhance the efficiency of the complement and macrophage activity against disease organisms. Once exposed to a specific disease organism, the B-cells display a “memory” of that organism and can respond to future challenges much more rapidly. The B-lymphocyte/antibody immune response is responsible for the protection afforded by vaccinations, in which a weakened or killed bacteria or virus is introduced into the body, allowing the “memory” capabilities of the B-cells to be activated and readied to produce antibodies if the B-cells detect the disease challenge in the future.

The B-lymphocyte/antibody immune response primarily prevents the disease organism from entering and damaging the target cells of the chicken. However, if the immune response was not able to prevent this from occurring, the next response by the acquired immune system is the production of T-lymphocytes. Depending on the specific type of T-cell, these cells can attack the organism directly, enhance the function of other cells involved in immune function (e.g., B-cells and macrophages) and kill infected cells when required.

When a chicken is exposed to a disease organism and produces antibodies itself, this is called “active immunity.” When a chick is hatched, the hen provides antibodies through the egg. Mammals secrete antibody-rich colostrum through the milk to their newborns. Obtaining pre-made antibodies is termed “passive immunity.” New feed additives are available which furnish egg-derived antibodies to livestock and poultry, and have been demonstrated to provide protection against many disease organisms. In fact, hens are such efficient antibody factories that egg-derived antibodies are becoming the mainstay for research and innovative immune therapy in humans. Certain vaccination programs for poultry are timed so that they are administered after the maternal antibodies have diminished somewhat, so that the chick’s B-cell’s are stimulated into producing antibodies and active immunity to the pathogen. If the vaccination is administered after the maternal antibodies have severely diminished, a reaction to the vaccination is possible.

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