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Structure and Function of the Immune System in Dogs

By: Virginia Wells

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What Is the Immune System?

The immune system is a complex network of specialized cells and organs designed to defend the body against bacteria, viruses, toxins, parasites and any foreign material that invade the dog's body. Millions of different types of immune cells pass information back and forth, which results in a protective system that is always ready to produce an immune response that is fast and effective. The immune system is also a component of the lymphatic system.

Where Is the Immune System Located?

The organs of the immune system are located throughout the dog's body. They are called lymphoid organs because they are frequently the site of growth, development and deployment of lymphocytes - white blood cells that are key operatives of the immune system.

Important components of the immune system are concentrated in the blood, thymus, lymph nodes, bone marrow, spleen, lungs, liver and intestines. When an infection starts in a location that has only a few components of the immune system, such as the skin, signals are sent throughout the body to call in large numbers of immune cells to the site of the infection.

What Is the General Structure of the Immune System?

The organs of the immune system are connected with one another and with other organs of the body by a network of lymphatic vessels similar to blood vessels. Immune cells, proteins, and sometimes foreign particles are carried through these vessels in lymph, a clear fluid that bathes the body's tissues. Various components of the immune system are also linked by the circulatory system.

The major components of the immune system include:

  • Lymph nodes. These are small bean-shaped structures lying along the course of lymphatic vessels in particular sites such as the neck, armpit and groin. They filter and trap antigens (the portion of a virus or bacteria that causes an immune response) that arrive at the lymph nodes from the lymphatic vessels and the blood stream.

  • Cells of the lymphocyte portion of the immune system. These cells may be divided into T cells and B cells. T-lymphocytes are initially processed by the thymus gland and are responsible for cellular immunity (the recruitment of other white blood cells to combat infection). B-lymphocytes receive their name from the Bursa of Fabricius, the area in the intestine of birds where these lymphocytes are initially processed. This Bursa does not exist in animals, and most B cells arise in the bone marrow of animals. B-lymphocytes are responsible for making antibodies that are proteins used to fight infections and foreign material. Both of these cells are widely dispersed in the body.

  • The spleen. This organ is located in the upper left quadrant of the abdomen. It filters and traps antigens directly from the blood stream.

  • Bone marrow. Marrow consists of connective tissue, the cells of which form a delicate meshwork within the marrow cavity. The marrow cavity is located in the center of several bones in the body, particularly the long bones. The bone marrow is the site of production of many white blood cells.

  • The thymus. This gland is located in the front part of the chest, just in front of the heart. It is largest in the young animal when the development of the immune system is at its most active, and it shrinks in size as the animal matures.

  • Leukocytes or white blood cells. A variety of white blood cells exist, and each has a special function in the immune system. Some are designed to react primarily to bacteria and inflammation, others react more to parasites and foreign material, and others assist the lymphocytes in producing antibodies.

  • Antibodies. Antibodies are specialized serum proteins produced by B cells in response to antigens. Antibodies are also called immunoglobulins. The body produces several classes or types of immunoglobulins.

    What Are the Functions of the Immune System?

  • Recognition of foreign substances. Foreign substances that invade the body are called antigens. The immune system has the ability to distinguish between "self" cells (cells of its own body) or "nonself" substances (foreign substances). Every cell in the body carries a molecule that identifies it as "self," so that the immune system does not attack its own tissues.

  • Protection. Adequate functioning of the immune system provides protection from infectious diseases or other invaders. Antigens may be microorganisms that cause infectious diseases, chemical substances, drugs, certain proteins, transplanted tissues or organs donated from another individual . The immune system may protect the individual from the development of cancer.

    Types of Immune Responses

    When exposed to a foreign substance or infectious agent, the immune system mounts two major immune responses, called nonspecific immunity and specific immunity. These responses occur in tandem and influence each other.

  • Nonspecific immunity. This type of immunity is present in all immunocompetent individuals at birth. It does not require a previous encounter with the offending substance, and it is active only for a transient period of time. It includes the protective barriers of the body, such as the skin and the mucous lining of the stomach.

    There are two main components of nonspecific immunity. One component is the phagocyte system, whose function is to ingest and digest invading microorganisms. The white blood cells primarily involved in phagocytosis are the neutrophils, monocytes and tissue macrophages. Another component is the natural killer (NK) cells, whose function is to kill some tumors, microorganisms and virally infected cells

  • Specific immunity. This type of immunity develops after the body has been exposed to a foreign substance. Specific immunity relies upon the body learning about the substance, adapting and responding to the substance, and then recognizing or remembering the substance when exposed to it again. The cellular component primarily responsible for specific immunity is the B-lymphocyte, and the specific response is the production of antibodies (immunoglobulins) against the substance.

    Each B cell is programmed to make one specific antibody. When a B cell encounters its triggering antigen, it stimulates many large plasma cells (another form of white blood cell). Each plasma cell is like a factory for producing that one specific antibody.

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