Fowls, like all animals, have very strong, built-in defences (immunity) against diseases that are caused by invasion of the body by various microorganisms and toxins (collectively called ‘antigens’). These defences include:

  1. The skin: The skin is a barrier against invasion of the body by microorganisms. It is only when the skin barrier is broken that invasion through the skin can occur.
  2. Mucous membranes: These linings of the digestive tract, respiratory tract and other body systems provide a good barrier against invasion by microorganisms. If anything should harm the mucous membranes invasion may occur. An example is when diets deficient in vitamin A cause damage to the mucous membranes which results in an increase in the incidence of infectious disease.
  3. The immune system: Notwithstanding the efficiency of the skin and the mucous membranes as defence mechanisms, microorganisms often find their way into the body. Many of these are harmless, while others cause disease, usually specific diseases caused by specific organisms.

The primary role of the immune system is to recognise organisms and substances that are considered foreign, or “non-self” (antigens), that are able to enter the body. The immune system initiates and manages the appropriate physiological responses to neutralise and/or eliminate these “non-self” organisms and substances. A variety of mechanisms are employed to achieve this goal, including inactivation of biological agents, lysis (rupture) of foreign cells, agglutination (clumping) or precipitation of molecules or cells, or phagocytosis (engulfing and inactivating) of foreign agents.

For an immune response to be effective, the right mechanism, or combination of mechanisms, must be activated. However, for each species there are many diseases for which immunity does not exist. Also, under certain circumstances, these normally protective responses can result in significant tissue damage, which leads to immune-mediated diseases. The immune system is a highly complex physiological system that is yet to be fully understood. New discoveries are still being made regarding the role of different organs and physiological compounds in immune responses and this section contains only a brief and simple description of the major components of the immune system and the immune response.

Whether or not a bird develops a disease after such an invasion will depend on how well it fights the invasion, that in turn depends on:

  1. The bird’s condition, state of wellbeing and level of immunity
  2. The number of the invading organisms, called ‘the challenge’
  3. The virulence or strength of the invading organisms

Innate immunity

This refers to the natural or inherited ability to resist disease and the involved mechanisms that come into play immediately or after a few hours that an antigen has appeared in the body. Included in this type of immunity are a number of non-specific disease response mechanisms:

  • Genetic factors: birds may lack the receptors required by disease organisms in order for them to be able to infect. For example, some strains of chickens are genetically resistant to the lymphoid leukosis virus.
  • Body temperature: the high body temperature of the chicken precludes many diseases. Blackleg disease of cattle is not normally a problem in poultry because poultry have a higher body temperature at which the disease-causing bacteria cannot survive. However, if the body temperature of the chicken is lowered, this disease may occur.
  • Anatomic features: many disease organisms cannot penetrate intact body coverings (skin and mucous membranes) or are trapped in the mucus secretions. Some nutritional deficiencies (such as biotin deficiency), injury or infectious diseases compromise the integrity of the body coverings, which allows penetration by disease organisms.
  • Normal microflora: the skin and gut normally maintain a dense, stable microbial population. This stable microflora prevents invading disease organisms from gaining a foothold in the bird. Improper use of antibiotics or poor sanitation can disrupt the balance of the microflora.
  • Respiratory tract cilia: parts of the respiratory tract are lined with fine hair-like protrusions, called cilia, which remove disease organisms and debris. If the air in the poultry house is of poor quality due to high levels of dust or ammonia, the ciliary system may be overwhelmed and become ineffective.
  • Immune system cells: are the cells that attack foreign cells in the body and are activated by the chemical properties of the antigens, these cells include:
    • White blood cells (WBCs) or leukocytes: WBCs move freely in body, interacting with and capturing cellular debris, foreign particles, and invading microorganisms. Most of the leukocytes involved in the innate immune responses are not capable of reproducing themselves by dividing, and are produced by multipotent hematopoietic stem cells residing in the bone marrow.
    • Mast cells:The innate immune cells residing in connective tissues and mucous membranes are known as mast cells. These cells are involved in wound healing and defence against pathogens. The substance released by activated mast cells “histamine” also initiate the recruitment of neutrophils and macrophages.
    • Phagocytes: The immune cells which kill the pathogens by eating or engulfing them are known as phagocytes. These cells are always patrolling the body and searching for pathogens, but are also reactive to some highly specialised molecular signals generated by other cells, known as cytokines. Macrophages, neutrophils, and dendritic are the phagocytic cells of the innate immune system.
    • Natural killer cells (NK): The cells of innate immune system that do not directly attack pathogens are called NK cells. These cells act by invading the infected host cells, such as tumour cells or virus-infected cells.

Other factors involved in the effectiveness of innate resistance include adequate nutrition, stress, age, inflammatory processes and metabolic factors.

Microarray technology helps scientists understand the genetic basis for a bird’s response to disease or vaccination

Adaptive immunity

The adaptive immunity -which is also known as acquired immunity- is sort of more complicated than the innate immunity. In such a way that, first the foreign cells or antigens should be recognised, and then the adaptive immune system creates and recruits an army of immune cells which are specifically designed to attack that antigen. Adaptive immunity also creates “immunological memory” once an initial response to a specific pathogen has been triggered, which can enhance future responses to that specific pathogen and act more efficiently. These antigen-specific immune response are provided by immune cells or antibodies (proteins produced by immune cells that bind with and inactivate antigens) that are produced in response to exposure to an antigen. These cells and proteins circulate through the body in the blood.

The immune cells or antibodies of the adaptive immune system are T and B lymphocytes. These antibodies are very small, very special proteins (globulin proteins) that animals release into their circulatory system,  which fight foreign invaders e.g. invading viruses and bacteria.

The adaptive immunity is specific to an antigen and can take one of two forms:

  1. Passive immunity: this is achieved when antibodies are transferred to the individual either from the mother (such as during egg formation in poultry or via colostrum ( antibody rich milk produced for newborn babies) in mammals) or by provision of antiserum (blood serum containing antibodies) either orally or through injections.
  2. Active immunity: this refers to an active immune response in a bird as a result of recovery from the disease or by response to a vaccine. The bird produces its own immune cells and/or antibodies to provide protection.

Passive immunity

With passive immunity, the immune system of the recipient is not stimulated, therefore the recipient will not produce its own immune cells or antibodies and it does not have any immune “memory” of the antigen. The newly hatched chicken is unable to mobilise its own immune system in time to fight invasion during the first few days (up to a week) of its life. To overcome this problem, it receives antibodies from its mother via the egg. These antibodies are specific for all of the disease causing organisms that the mother has experienced or has been vaccinated against. These antibodies are usually short lived but survive long enough to provide a defence system for the first few days of life while the chick’s own defence system becomes operative.

The level of immunity passed on by the mother is about the same as she carries herself. However, by the time the egg hatches this has dropped to about half. It is important, therefore, that the antibody level of the breeder hen be boosted frequently to ensure that her antibody levels do not decline too much, which would lead to a reduction in her offspring’s ability to fight invasion at the critical period just after hatching.

The flock manager must be aware of the maternal antibody levels in the chick in order to appropriately schedule vaccinations. If young chickens are vaccinated when maternal antibody levels are still high, the vaccine may be neutralised excessively by the maternal antibodies, which will result in a reduced immune response. Conversely, if vaccinations are delayed and maternal antibody levels drop too low, a severe vaccine reaction may result and the chick will be left vulnerable to disease prior to vaccination.

Active immunity

The immunity the chicken obtains from its own immune system is called active immunity. This system must be activated or triggered to produce the antibodies or other weapons that fight invasion. This defence mechanism has the ability to remember specific invaders, which results in quicker and more effective responses to repeat exposure to an antigen, and produces defences that are specific for that invader (they will fight that particular invader only).

Active acquired immunity is divided into non-cellular (humoral) and cellular components:

The non-cellular (or humoral) immune response:

The non-cellular component includes antibodies and the cells which produce them. Antibodies are short-lived and are specific for the antigen to which they attach. For example, the antibody against Newcastle disease virus will attach only to the Newcastle disease virus, not to the infectious bronchitis virus.

When a disease organism enters the body, it is engulfed by a phagocytic-type cell called the macrophage. The macrophage transports the disease organism and exposes it to the B-lymphocytes (B-cells). These cells are produced in the chick embryo in the liver, yolk sac and bone marrow. The cells move to an organ called the bursa of Fabricius (BF) after 15 days incubation through to 10 weeks of age. When an antigen is encountered, the BF programs these cells to attach to the antigen. The programmed cells then move to the blood, spleen, cecal tonsils, bone marrow, Harderian gland and thymus. Destruction of the BF in a chicken at a young age by Gumboro (infectious bursal) disease or Marek’s disease prevents programming of B-cells for life. Thus, the affected chicken will not be able to respond to diseases or vaccinations by producing specific antibodies.

The B-cells then respond by producing antibodies specific to the disease organism. A five-day lag period occurs because the B-cells must be programmed and undergo clonal expansion to increase their numbers. The B-cell system also produces ‘memory’ cells that remember infections. These have a relatively long life and stimulate the antibody producing cells to quickly release large amounts of antibodies specific for particular disease organisms that the bird has previously experienced, which results in a quicker and more effective immune response than the initial response.

Antibodies do not have the capability to kill disease organisms directly. Antibodies perform their function by attaching to disease organisms and blocking their receptors. The disease organisms are then prevented from attaching to their target cell receptors in the chicken. For example, an infectious bronchitis virus which has its receptors covered with antibodies will not be able to attach to and penetrate its target cells (the cells lining the trachea). The attached antibodies also immobilise the disease organism which assists their destruction by macrophages.

The cellular immune response:

The cellular component of the immune response includes all the cells that react with specificity to antigens, except those associated with antibody production. The cells associated with this system, the T-lymphocytes (T-cells), begin as the same stem cells as the B-cells. However, the T-cells are programmed in the thymus rather than the BF.

A range of T-cells are produced to perform various roles. Some T-cells act by producing chemicals called lymphokines (see below); others directly destroy disease organisms; some T-cells act to enhance the response of B-cells, macrophages, or other T-cells (called helper T-cells); while still others have the opposite effect and act to inhibit the activity of these cells (suppressors). The cellular system was first identified when it was shown that chickens with damaged BF, which were therefore unable to produce specific antibodies, could still respond to and eliminate many disease organisms.


Lymphokines are soluble proteins that assist in activating other components of the immune response. Over 90 different lymphokines have been identified.

The lymphokines act by:

  1. Reacting with white blood cells to increase their ability to battle the foreign invasion
  2. Increasing the rate of lymphocyte production
  3. Breaking down cells (damaged body cells, bacteria)
  4. Performing other related functions

Vaccines and the immune response to vaccination

If the chicken is exposed a second time to the same antigen, the response is quicker and a much higher level of immune cell and antibody production occurs (memory). This is the basis for vaccination. There are a number of types of vaccines, with advances in technology resulting in the continuing development of new vaccine types. Some types of vaccines include:

  • Live and live-attenuated vaccines: Live vaccines contain either low doses or doses of mild forms of the disease organism. Live-attenuated vaccines contain living disease organisms that have been treated in some way to reduce their ability to cause disease while still causing an immune response. Both of these vaccines contain living organisms that are able to infect and multiply in the host and this enhances the strength and duration of the immune response.
  • Killed (or inactivated) vaccines: Killed vaccines contain high doses of the killed disease organism. Killed vaccines generally result in a weaker and shorter immune response than live vaccines due to their inability to infect and multiply in the host.
  • Sub-unit vaccines: These vaccines contain doses of purified antigens extracted from the disease organism.
  • Recombinant vaccines: These vaccines are produced by incorporating the DNA of the antigens that stimulate a disease response to a disease organism into a vector (or carrier), such as a harmless virus, which is then used as a live vaccine.
  • DNA vaccines: These vaccines contain purified DNA of the antigens that stimulate an immune response to a disease organism.
  • Conjugate vaccines: These vaccines are used to elicit an immune response to an antigen that is normally able to evade detection by the immune system. They contain the antigen bound to a compound, such as a protein, to form a complex that is detectable by the immune system.

Vaccines sometimes contained compounds called adjuvants. Adjuvants act to increase the strength of an immune response to a vaccine and are commonly included in killed vaccines to boost their efficacy.

Vaccine production requires careful handling

Maintaining good bird health

To maintain flocks of poultry in good health managers should ensure that:

  1. The challenge is kept as low as possible.
  2. The flocks’ ability to fight infection is kept as high as possible:
    • By ensuring the flock is as healthy as possible.
    • By regular stimulus or boosting of the immune system to produce appropriate antibodies and lymphokines to fight invading micro-organisms.

In some cases the birds’ immune system requires frequent boosting to be effective against an antigen, while in other cases, less frequent stimulation is required. In some situations it is possible to depend on natural infection to trigger the immune system to produce the antibodies. However, in these cases it is necessary to take precautions to ensure that the infection does not get out of control.

Coccidiosis and fowl pox

The prevention of coccidiosis is a good example of this technique. Many managers allow the flock to become infected with coccidiosis and then treat the outbreak. They assume that the outbreak has triggered the surviving birds’ immune system to produce the antibodies to coccidiosis. In some situations, it is common practice to boost the immunity level by natural infection once the birds have achieved a good level of immunity by vaccination. The system used to combat fowl pox disease by some managers is a good example of this technique.

Therefore, when the immune system is triggered the response is not the same on every farm or for every disease. There are so many factors that influence the different responses that each situation must be assessed independently and a prevention program devised that best fits the situation. An integral part of this program is the need to use a high standard of quarantine and hygiene to ensure that the challenge is kept as low as possible.

Marek’s Disease

Marek’s Disease is a special case. When the birds’ are vaccinated against this disease, they remain carriers of the virus and continue to shed it for the rest of their life. The vaccine only prevents the development of symptoms. It takes about four days for the birds to develop their response to the vaccine and even birds’ vaccinated at day old will develop Marek’s Disease if they are challenged before the response to the vaccine has developed. Some hatcheries now use a system of vaccinating embryos on the 17th day of incubation so that when the chickens hatch 4 days later they will carry a high immunity right from the first day.

The Marek’s Disease virus is found in the dander and other debris from the body of the bird and survives there for many months, therefore it is of the utmost importance that the birds be excluded from this material during their first week of life in the brooders. This can only be achieved when a very high standard of hygiene and shed cleaning is practised.