This is a very simplified attempt to explain the broader aspects of immunity to pet and bird owners.

Immunity is a medical term that describes a state of having adequate defences to avoid infection or disease or other unwanted biological invasion. When an antigen (a protein molecule that forms part of a pathogen) enters the body, an immune reaction is triggered.  A pathogen is a micro-organism such as a bacterium or a virus that is capable of causing disease.

In the body the reaction to the antigen takes place in the myeloid tissue contained in bone marrow and in the lymphoid tissues such as the thymus, the lymph nodes, and the spleen. In birds the bursa of Fabricius is a particularly important organ in the immune system.

It is in the bone marrow that a range of white blood cells are manufactured. These include several different types of phagocytes (literally meaning “eating cells”) amongst which are the macrophages.  These are large white blood cells circulating throughout the body in the bloodstream that act as the scavengers of the body. In an immune reaction, one of their functions is to gobble up the invading pathogen – one single macrophage is capable of destroying 100 bacteria! The body also produces a range of protein molecules called antibodies (immunoglobulins) that will specifically react with the pathogens to destroy them. The circulating macrophages, together with other types of white blood cells and the immunoglobulins form part of the humoral immunity – immunity in the blood stream. By measuring the levels of immunoglobulins, the level of this type of immunity can be gauged.   

The immune reaction that takes place in the lymphoid tissues produces a variety of special cells, some of which are responsible for the second type of immunity called local or cell-mediated immunity. An example of this type of immunity is where after having recovered from a cold, a person’s nasal tract and throat would have local immunity to the specific virus that originally caused the cold. There is no way to measure this type of cell-mediated immunity but it remains an important component of the body’s overall immune reaction.

HOW CAN OUR KNOWLEDGE OF IMMUNITY BE USED TO OUR ADVANTAGE?

In order to protect ourselves, our pets and our birds from infections, an understanding of the different types of immunity allows us to take appropriate action, depending on the circumstances of the threat.    

 PASSIVE IMMUNITY

 Innate immunity is a natural immunity to pathogens that normally affect other species. For example, neither cats, nor humans nor pigeons contract canine distemper.  

 In mammals, natural passive immunity is found in newborn youngsters. It is short-lived (a few weeks at best) and can be acquired either through the placenta whilst the foetus is still in the mother’s uterus, or more importantly, through the colostrum. Colostrum is the thick, antibody rich milk that a mother produces in the first 24 hours after giving birth. In order to protect a newly born youngster against the myriad infections to which it will be exposed, it is vital that the puppy or kitten drink as much colostrum as possible during these first 24 hours. After that the newborn’s digestive system will start to break down the antibodies contained in the mother’s milk, thus losing the benefit of being able to absorb the whole antibody as it would have done in the first 24 hours. This type of maternally derived immunity will usually only last for about 8 to 12 weeks. By this time the youngster’s developing immune system will be able to manufacture its own antibodies to specific infections (or vaccine antigens).

 In birds, natural passive immunity is acquired from swallowing the antibody-rich amniotic fluid during the hatching process and by absorption of the anti-body rich egg yolk after hatching. Maternal immunity in birds lasts up to ten days. The bird’s bursa of Fabricius becomes the most important organ for the first 6 weeks of a bird’s life. Thereafter the bursa shrinks in size and the other lymphoid tissues take over the role of manufacturing the cells that defend the body from unwanted infections.

 Artificial passive immunity occurs when antibodies are injected into the animal or bird because they are required immediately. Examples of this would be the use of an anti-tetanus injection following a deep wound, or the injection of anti-snake bite serum into a patient following a snake bite.

ACTIVE IMMUNITY – it generally takes about two weeks for the body to develop active immunity. This is why for example it takes about two weeks to get over a cold. The cold goes away because the immunity built up to the cold-causing virus eventually removes the offending virus from the body.   

 Natural active immunity occurs where infection with a natural disease has stimulated the body to build up an immunity to the disease. An example here would be a child that has had measles. The attack of measles goes away because of the immunity that has been built up during the course of the disease. Following recovery, the child will be protected from further attacks of measles because of the immunity that the body has developed. In this particular case the immunity will be life-long.

 Artificial active immunity develops after a vaccine has been administered. The immune system develops an immunity to the specific antigen contained in the vaccine, thus protecting the patient from infection against the natural disease. This type of immunity is seldom life long but will usually protect the patient for quite a long period of time, perhaps for a year or two. Booster vaccinations from time to time increase the gradually declining level of immunity, making it possible to protect an animal throughout its life.

PRACTICAL EXAMPLE OF OUR ABILITY TO USE THE ABOVE KNOWLEDGE TO GOOD ADVANTAGE

Certain toxins such as those produced by tetanus bacteria stimulate an immune reaction. However, because the speed with which the toxin will kill a patient (within a couple of days) suffering or potentially suffering from tetanus, urgent treatment in the form of tetanus antitoxin (artificial passive immunity) is required. There is no time (remember it takes 2 weeks) for the body to develop its own antibodies. A better approach is to vaccinate (artificial active immunity) people or animals at risk with a tetanus vaccine so that the body already has its own supply of antibodies in the event of infection by tetanus bacteria.

 WHAT HAPPENS WHEN THE IMMUNE SYSTEM GOES WRONG?

 There can be many causes of immunodeficiency. Anything that can affect the body’s immune reaction at any point along the immune response chain can lead to immunodeficiency. For example, in certain types of leukemia, the bone marrow is affected with the consequence that no white blood cells, including the macrophages, are manufactured as part of the immune response. This is an example of where a bone marrow transplant might help by replacing the affected bone marrow with healthy bone marrow tissue. 

 Infections like HIV (human immunodeficiency virus) attack the lymphoid tissues and some of the special cells normally involved in the immune reaction, thus decimating the body’s normal defense mechanisms. This destroys the body’s ability to develop immunity to other infections such as tuberculosis. AIDS patients usually die from these other infections, not from the HIV virus.

 In poultry, infectious bursal disease virus infects the bursa thus destroying the bursa’s ability to manufacture B (bursal produced) lymphocytes which play a vital role in the bird’s humoral immune system. This opens the way for all sorts of other infections like Newcastle disease.

 Allergies are also part of the immune reaction of the body. Allergens such as the proteins contained in beestings stimulate the same sort of immune reaction in the body as the antigens discussed above. In the case of allergies, the body becomes over sensitive to the allergen, leading to potentially severe allergic reactions should the body be exposed to that particular antigen again.

 This article has briefly touched on some of the aspects of the complicated subject of immunity. We hope that it has been of interest.