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Anatomy and Physiology

The immune system itself is a complex network of cells, organs, and special chemicals. No one division overshadows another; each team member relies on the others for support. In this way, they all work in unison toward a common goal.

Cells of the Immune System

Special cells, called stem cells, located within the bone marrow give rise to all the cells of the immune system. The cells that are produced by these stem cells are referred to as white blood cells. This general category comprises numerous types, each serving distinct functions.

The neutrophil functions to gobble up bacteria that gain entrance into the body. Also assisting in this function are white blood cells called macrophages. These cells usually come after the neutrophils are already engaged. In addition to bacteria, macrophages also have the capability to eat viruses, fungal organisms, and foreign matter.

When a dog or cat is vaccinated, special immune cells called lymphocytes are stimulated. B lymphocytes are responsible for producing actual antibodies in response to the vaccine or foreign organism; T lymphocytes don’t produce antibodies per se, yet they assist the B lymphocytes in doing so, and help modulate the immune response.

They also have the ability to attack and kill cells within the body that are cancerous or infected with viruses. Both B and T lymphocytes are said to possess “memory”—that is, they remember the various organisms and invaders that they’re fighting against.

That way, if they show up again at a later date, they will be attacked without hesitation. Yet even with memory, this response can become slower and weaker over time if the immune system remains idle. This is why certain vaccination boosters are needed periodically.

Another important lymphocyte of the immune system is called the natural-killer cell, which searches for and destroys tumor cells and cells infected with viruses. Unlike their T-cell counterparts, naturalkiller cells do not possess memory, yet at the same time, few of them require a previous exposure to a foreign agent to respond effectively.

Organs of the Immune System

The organs of the immune system include the bone marrow, the thymus, the spleen, and the various lymph nodes and aggregates of lymph tissue spread throughout the body.

Bone Marrow

As mentioned before, all cells of the immune system originate within the bone marrow. Many stay put and undergo maturity right where they are; other cells are shuttled off to the thymus.


The thymus is an organ located in the neck region of young animals. As that individual ages and the immune system undergoes a mature development, the thymus gland gradually disappears. It is in this organ that most of the T cells undergo their maturation.

Lymph Nodes and Tissue

From the thymus and the bone marrow, the cells of the immune system are then shipped to the front-line defenses, including the lymph nodes, tonsils, and other lymph tissue lining the gastrointestinal and respiratory tracts.

This latter tissue, owing to its strategic location, provides a first line of defense against organisms that try to gain entrance into the body. B lymphocytes in this tissue produce special antibodies that coat the surface of the tract, and block such access.

Lymph nodes and tissue are responsible for filtering the body’s blood and lymph for foreign agents and cells. Lymph is a special type of fluid that circulates throughout the body within its own separate channels or vessels, called lymphatic vessels.

Fats that are absorbed via the intestinal tract enter into this lymphatic system, as do lymphocytes on their way to and from the front-line defenses.

The Spleen

The spleen is an organ most have heard about; its various functions include filtering blood and providing a storehouse for blood cells.

Chemicals of the Immune System

Special chemicals produced by the cells of the immune system serve to assist them in their protective function. Among those chemicals generated are interferon, interleukins, and complement. Interferon is a protein that is produced and released by cells that have been invaded by or come in contact with a virus.

Released within 2 hours after the cell is invaded, it acts as a messenger to surrounding healthy cells and stimulates the immune system to respond. It even has antitumor effects, preventing tumor cell replication in some instances.

Interleukins are chemicals produced by macrophages that help control and modulate the activity of the T cells during an immune response. Interference with the release of interleukins, which can occur with many viral diseases, can lead to immunosuppression.

Complement is a special protein produced by the body that attaches itself to the surface of antibodies. When these antibodies bind to a bacterium or infected cell, the complement serves to “burn” a hole in the cell membrane, leading to the cell’s destruction.

In some instances, complement doesn’t need the help of antibodies to fulfill this function.


An antigen is defined as any substance capable of eliciting an immune response. Infectious organisms—such as bacteria, foreign matter, and even tumor cells—have antigens within their makeup and lining their outer surfaces. Since the body does not recognize such an antigen as one of its own, it mounts an immune response against it to try to eliminate it.

On initial exposure to an antigen, B lymphocytes start to divide and differentiate into their antibody-producing form. It might take up to 7 days before antibody production can be achieved.

Even then, production is only moderate, and adequate levels generally last only about 3 weeks. In the meantime, however, other immune components, such as neutrophils, macrophages, and killer cells, are called in to fight off the invader.

If the invader is a tumor cell, foreign body, or an organism that lives and multiplies within body cells (such as viruses do), then the T lymphocytes start to multiply and prepare themselves for battle as well.

As with the B lymphocytes, they are specific for each antigen; that is, a lymphocyte that responds to one type of antigen will not respond to any others. As a result, each different antigen that enters the body will stimulate its own group of antagonistic B and T lymphocytes.

Following this initial exposure, the lymphocytes that have been primed to the antigen retain “memory” of the experience. Sent to the front-line defenses, they simply wait for the antigen to show up again. If it ever does, the lymphocytes are ready for it, without the 7-day lag time.

Antibodies are produced in high levels almost immediately, and the T cells are primed and sent into action with minimal delay.