The function of white blood cells (also called leukocytes) is to defend the body against infection. There are 2 main types of these formed in the bone marrow: 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 by surrounding and destroying them. There are two types of phagocytes: granulocytes and monocytes.
Granulocytes protect against bacteria, fungi, and parasites. 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 in dogs. Eosinophils and basophils are involved both in protection against some parasites and in the response to allergy in dogs. Monocytes travel from the dog's blood to the dog's tissues where they become large cells called macrophages that consume foreign particles and cell debris.
As with red blood cells in dogs, the production and number of phagocytes are tightly regulated by chemical messengers of the dog's blood, including interleukins (chemicals found in a dog's white blood cells that stimulate them to fight infection in dogs). Unlike your dog's red blood cells, which remain circulating in the blood, the phagocytes use the blood's circulatory system as a pathway to the dog's tissues. Because of this, the number of phagocytes in the blood can provide an indication of circumstances in the dog's tissues. For example, the number of neutrophils increases when inflammation is present anywhere in a dog's body, an abnormal response, such as a low number of circulating white blood cells due to marrow failure, infections, drugs, or toxins, can lower resistance to bacterial infections. Finally, those elements that produce phagocytes may become cancerous, resulting in a disease in dogs called myelogenous leukemia.
Lymphocytes are white blood cells that produce antibodies to attack infectious organisms in the dog's body. They are also responsible for rejecting foreign tissue or cancer cells. Lymphocyte production in mammals begins in the bone marrow. Lymphocytes then become 1 of 2 types: T cells or B 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, including fighting off viral infections and cancers. Most T cells remain in circulation, but some are also present in the spleen and lymph nodes. The dog's B cells are responsible for producing antibodies that coat invading organisms or foreign substances.
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