This is a very simplified attempt to explain the broader
aspects of immunity to pet 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 animal’s body the reaction to the antigen takes
place in the myeloid tissue contained in bone marrow and in the lymphoid tissues
such as the lymph nodes, the thymus and the spleen.
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 and our pets 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
contract canine distemper.
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).
Artificial passive immunity occurs when antibodies
are injected into the animal 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.
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
This article has briefly touched on some of the aspects of
the complicated subject of immunity. We hope that it has been of interest.