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
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.
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
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.