The Canine Immune System

Bacteria, viruses, fungi, foreign proteins, cancer are all organisms or conditions which constantly pose a threat to the canine body. Like all species of organisms, the canine body is equipped with an elaborate system of defense, known as the immune system, designed to protect it from these infectious enemies.

There are two major components to immunity. The first component called "recognition" occurs when a foreign agent invades the body for the first time. The immune system recognizes the agent as foreign and within a short time, a series of reactions begin which eventually destroy the invader. Because it takes time for the immune system to launch its defense, first-time invaders usually will produce symptoms of illness, and severity will depend on the extent of exposure and invasiveness of the enemy agent. However, once the first-time invaders are destroyed, if that particular enemy agent attempts to invade at a later time, the immune response will occur much more rapidly and the body will experience few or no symptoms before the agent is destroyed.

The second component of the immune system is called "discrimination". The immune system must be able to differentiate the normal tissues and fluids that make up the canine body from the invading agent. To facilitate this, the cells of the dog's own body have a unique set of molecules on their surface which allows the dog's immune system to recognize them as its own cells. However, the molecules on the surface of an invading agent, called antigens, will be different and will, therefore, allow the immune system to recognize the agent as foreign and initiate a defense.

Therefore, for normal immune system function, both of these components, recognition and discrimination, are vital for the survival of the dog.

How the Immune System Works

When a foreign agent gains access to the body of the dog, the intrusion is detected through the body's security network called the lymphoid system. Lymph nodes are strategically located to guard portal entries into the body. An enemy invader will eventually reach the circulation and be filtered out through the lymph nodes or the spleen. In the lymph nodes, white blood cells called macrophages, surround and degrade the foreign agent and eventually expose antigens. The immune system then responds to the antigens in two ways. B lymphocytes, cells originating in the bone marrow, have proteins on their surface which will bind to the antigens. Binding, in turn, activates the B lymphocyte to mature into a plasma cell that multiplies and then is released into the blood circulation. Once circulating through the body, the plasma cells synthesize and secrete specific antibodies that target and destroy all invaders displaying that particular antigen. Once the infectious material is destroyed, the mature B lymphocyte, or plasma cell, remains in circulation as a "memory cell". If the body becomes invaded again by the same foreign agent, the memory cell produces antibodies to the antigen so rapidly that the infectious agent does not have an opportunity to multiply and produce symptoms of infection in the dog.

In addition to the B lymphocyte, the immune system is composed of another cell type which can recognize and bind to antigens. However, these cells do not secrete antibodies. Instead, T lymphocytes that mature through the thymus gland, have proteins on their surfaces called T cell receptors which may bind to the antigen. Additionally, these immune cells release certain biological factors that attract macrophages to the area of infection. There are three types of T cells involved in immunity: the cytotoxic or killer T cells bind to and destroy other cells which display antigens on their surface; the helper T cells which assist B cells to stimulate the growth and secretion of antibodies; and suppressor T cells which reduce B cell activity and thereby play a role in reducing the possibility of an autoimmune response.