Avian Immune System – The Police of The Chicken Body

Like all animals, chickens, as well as other poultry, have immune systems like mammals do. Below is an insight to the defence of the avian body!

Primary Defences

Innate Immunity

Acquired Immunity

  • Passive Immunity
  • Active Immunity
    1. Cellular Immunity
    2. Non Cellular (Humoral) Immunity

    Skin: The skin is the first, and primary defence. It forms a physical barrier which blocks harmful micro-organisms from entering. Therefore, invasion can only occur if the skin is broken.

    Mucous Membranes: As the name suggests, these are membranes, covered in mucous! They are found lining the digestive tract, respiratory tract and several other body systems. The mucous carries the micro-organisms away, and expels them, via nasal discharge, or out the other side in the faeces.

    Immune System: Finally, if the invaders have bypassed all the other defences, the immune system comes in. It consists of various organelles and cells which are primed to target anything alien. Many of these are harmless, but the ones that cause disease must be eradicated!

    The immune systems primary role is to recognise foreign bodies and neutralize or eliminate them. This is done by lysis (rupture) and agglugation (clumping together) of foreign bodies, or phagocytosis (engulfing and de-activating).

    Innate Immunity is one of natures tools. It is the natural, or inherited ability to resist infection or disease.

    • Some birds are naturally resistant to diseases like lymphoid leukosis because they lack the receptors that the lymphoid leukosis virus infects.
    • Chickens have a high body temperature, so diseases from other animals are usually not a problem, because the pathogen is killed at high temperatures.
    • Normally, the skin and digestive tract are riddled with naturally beneficial bacteria. These stop invaders from gaining a foothold
    • The respiratory tract has fine hairs, called cilia. These are washed over with mucous and any bacteria foolish enough to dare penetrate the tract will be washed away and expelled out of the nose.

    Acquired Immunity is a very effective type of immunity that the bird gains over its lifetime. White blood cells or leucocytes are found in the blood. These are the ”police” of the body, and they hunt the criminals! There are phagocytes, which phagocytose (engulf and deactivate) intruders, and lymphocytes which release antibodies (special globulin proteins) to bind to and inactivate the antigens.

    There are two main ways in which acquired immunity works…

       

    1. Passive Immunity – This involves immunity transferred from individual to individual, for example, from hen to chick through the egg. In mammals, a similar process takes place through colostrum (antibody rich ”first milk”).
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    3. Active Immunity refers to the immunity gained from acquiring a disease then defeating it. The  chicken keeps memory cells in the blood, so if the chicken is re-infected, antibody producion will occur so fast that the disease will not have time to cause symptoms.

    Passive immunity

    The first few days after hatching, a chick’s immune sytem is not functional, so it cannot fight invasion for itself. The solution to the problem comes through the egg. The mother passes on some antibodies which, although short lived, will guidde the chick through the first few days of life. These antibodies are from the mother’s acquired immunity, either from  vaccines or infection, then defeat of a disease.

    The level of immunity that is passed into the egg is similar to her own level, but after the 3 weeks of incubation, this drops to half. Therefore , it is valuable for the flock manager to keep the mother’s immunity levels high, to promote the health of her chicks. If vaccination of the chick is to be considered, remember that doing so too early could cause a subdued immune response, due to maternal antibodies attacking the vaccine, but if left too late, the chick will be open to disease, and may have an excessively high immune response.

    Active Immunity

    As I said above, active immunity is where the body fights off a disease, and keeps memory cells in the blood, so at the firt sign of re-infection,  antibodies are produced, and the disease cannot take a hold. This defence is effective, but only for specific antigens.

    Active immunity is divded into two parts, namely non-cellular (humoral) and cellular…

     

    Non-Cellular (Humoral) Immunity

    Non cellular (humoral) immunity involves antibodies, and the cells that produce them. Antibodies only exist for a short time, and are specific to their antigen. For example, the antibody for Infectious coryza, affects only the pathogen for Infectious coryza, and not the Infectious Bronchitis virus.

    When a pathogen enters the body, it is engulfed by a phagocytic WBC, known as a macrophage. It is then transported to and exposed to the B-lymphocytes (also known as B-cells).

    B-cells are produced (in the chick) by yolk sacs, the liver and the bone marrow. After 15 days of incubation, through to ten weeks of age, these cellsare moved to an organ known as the Bursa of Fabricius (BF). When required, the BF ”programs” the B-cells to attack the antigen with antibodies. The programmed B-cells then progress onto the spleen, the blood, the cecal tonsils, the bone marrow, the Harderian gland (in the eye) and thymus (an organ which ”educates” T-lymphocytes (T cells)).

    Destruction of the BF in chicks of a young age, due to Marek’s disease or Gumboro disease, means that the chick can never program B-cells, so cannot respond to most invasions, or vaccinations.

    When invasion occurs, the B-Cells release antibodies to fight the intruders. Macrophages also join the fight, coming to gobble-up the antigens which the antibodies have inhibited. After a war is over, the B-cells produce ”memory cells” which ”remember” the intruder, so should the intruder attack again, they can produce large amounts of antibodies, to generate a quicker and more effective response than the initial attack.

    Antibodies do not have the capability to kill disease directly, rather, they bind to the pathogens and inhibit their receptors. This means that they cannot locate, and bind to the target receptor.They also acts as a ”flag” for the phagocytes (macrophages) to detect invaders! Finally, though, it is up to the macrophages to engulf, and destroy pathogens.

    Cellular Immunity

    The cellular component of the immune response involves all cells that react to a specific antigen, except for those involved in antibody production (B-cells etc). The main players here are the T-lymphocytes (T-cells). These begin as the same stem cells as B-cells, but are programmed in the thymus rather than the BF.

    T-cells are produced and programmed to perform various functions, some T-cells assist B-cells and macrophages, these are known as helper T-cells, some produce lymphokines, others (directly) destroy invading organisms. Others still are known as suppressors, and inhibit the effects of macrophages and B-cells.

    This response was first discovered when it was found that birds with a damaged BF can still fight many infections and diseases!

    Lymphokines

    These are chemicals in the form of soluble proteins, that assist in activating other components of the immune response. Almost 100 different types have been identified!

    They work by:

    • Binding to WBC and increasing their ability to fight off invasion
    • Breaking down damaged, and invading cells
    • Increasing lymphocyte production
    • Performing many other related functions.