One of the most common reason we see dogs at our clinic is for skin issues. The underlying cause for many of these cases, yet certainly not all, is allergies.
First, let's define what allergies are:
Allergies are caused by a malfunction of the immune system. The immune system is made up of cells that are designed to protect the body against invading organisms like viruses and bacteria. The way this works in very interesting: the cells of the immune system have something like an "internal registry" that lists all the bodies proteins, or "self" proteins (which are by contrast, a normal part of the animal's body). These cells patrol the whole body through the blood vessels looking for any "foreign" protein, which is any protein that is not found in the aforementioned internal registry.
Whenever the immune system cells find a foreign protein, they start a reaction to destroy this invading protein. The reaction consists of increasing the blood flow to the area of the body where the invading protein was detected, to allow more immune system cells to reach the area and participate in combating the invader. Because of this increased blood flow, the area becomes red. Also, the yellow fluid in which all the blood cells float might leak to the area as well, which is the reason why you sometimes find dried yellowish material on your pet's skin.
Allergies take place when the body reacts to a "foreign" protein that does not belong to an invading organism like a virus or a bacteria, but rather comes from another source. There are three sources for these foreign proteins. These sources are:
1- Environment (plants, insects, ...etc)
2- Food (proteins are a main component of the animal food as well as ours)
3- Flea saliva
The question then becomes: "Why do some animals have allergies and others don't even though they're sharing the same space and the same food?
The answer is what we call the "Barrier theory". In simple terms, the foreign proteins do not alarm the immune system as long as these proteins do not come in contact with the immune system cells, which are in the blood.
Normal animals have intact barriers to prevent these two from coming in contact. Animals that have allergies have a defective barrier.
With environmental allergies, the animal's skin allows proteins from the environment to pass through the skin and reach the immune system cells, which in turn causes an allergy inflamation.
With food allergies, the animal's intestines allow the food proteins to come in contact with the immune system cells, which again, causes inflammation.
Another factor is the difference in the degree of
reaction to foreign proteins by the immune system of individual animals. To illustrate that, we should consider the third common cause of allergy, flea saliva. Whereas every animal that gets bit by a flea gets exposed to the proteins in its saliva, not all of them react with the same intensity and thus not all of them develop allergies.
In the next article we will discuss, how we diagnose and how we treat these different types of skin allergies.
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