The function of white blood cells (also called leukocytes) is to defend the body against infection. There are 2 main types of white blood cells: phagocytes and lymphocytes.
Phagocytes (from the Greek word meaning “to eat”) are cells in the bloodstream and tissues that surround and consume foreign particles, cell waste material, and bacteria. Their main function is to defend against invading microorganisms.
There are 2 types of phagocytes: granulocytes and monocytes. Granulocytes protect against bacteria, parasites, and fungi. Some types of granulocytes are involved in allergic reactions. Neutrophils are the most numerous of the white blood cells and are the first line of defense against bacterial invasion. Other granulocytes, known as eosinophils and basophils, are involved in both protection against some parasites and allergic reactions. Monocytes travel from the blood to tissues, where they become cells called macrophages, which consume foreign particles and cellular debris.
As with red blood cells, the production and number of phagocytes are tightly regulated by chemical messengers of the blood. Unlike red blood cells, which remain circulating in the blood, phagocytes use blood vessels as a pathway to the tissues. Because of this, the number of phagocytes in the blood can provide an indication of circumstances in the tissues. For example, the neutrophil number increases in the presence of inflammation. An abnormally low number of circulating neutrophils (due to marrow failure, drugs, or toxins) can lower resistance to bacterial infections. Finally, those elements that produce phagocytes may become cancerous, resulting in a disease called myelogenous leukemia.
Lymphocytes are white blood cells that recognize and defend against specific infectious organisms. They also reject foreign tissue and cancer cells. Lymphocyte production in mammals begins in the bone marrow. Lymphocytes then become T cells, B cells, or natural killer cells. Lymphocytes destined to protect cells from disease travel to the thymus (an organ located at the base of the neck), where they become T cells under the influence of hormones there. T cells are responsible for a variety of functions, especially fighting off viral infections and cancer cells. Most T cells remain in blood vessels, but some are also present in the spleen and lymph nodes. B cells produce antibodies that coat invading organisms or foreign substances, marking them for elimination by the immune system For example, antibodies can coat bacteria making it easier for phagocytes to consume them. Natural killer cells and some types of T cells (cytotoxic T cells, for example) destroy foreign material, whereas antibodies and other T cells (helper T cells) stimulate other parts of the immune system to do so. If lymphocytes are reduced or abnormal, the cat is immunodeficient and susceptible to a wide variety of infections.
Antibody molecules are called immunoglobulins. They include several classes, each of which has a different function. For example, one class (IgA) is commonly found in the lungs and intestines; another (IgM) is the first antibody produced in response to newly recognized foreign microorganisms; a third (IgG) is the main antibody in the bloodstream; and a fourth (IgE) is involved in allergic reactions.
Lymphocytes usually act appropriately to rid the body of foreign “invaders” that cause disease. An inappropriate response occurs when antibodies are produced against the body’s own cells, such as red blood cells. This can result in what are called autoimmune diseases (literally, immune diseases directed against the self), such as immune-mediated hemolytic anemia. Another inappropriate response of the immune system is allergy. When antibody-primed cells are exposed to an allergen, the allergic reaction may be mild (hives) or life-threatening (anaphylaxis).
Lymphocytosis, an increase in the number of circulating lymphocytes, may occur as a response to the secretion of epinephrine (a hormone also known as adrenaline). A reduction in the number of lymphocytes circulating in the blood may be caused by corticosteroid drugs or hormones secreted during times of stress.
Malignant tumors originating in a lymph node (lymphoma) or lymphoid leukemia can also occur.