Merck Manual

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Pet Owner Version

White Blood Cells of Horses


Susan M. Cotter

, DVM, DACVIM, Cummings School of Veterinary Medicine, Tufts University

Reviewed/Revised Mar 2019 | Modified Oct 2022

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 "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 2 types of phagocytes: granulocytes and mononuclear phagocytes. Granulocytes, mostly a type called neutrophils, protect against bacteria and fungi. Others known as eosinophils and basophils are involved in allergic reactions and defense against parasites. Mononuclear phagocytes travel through the bloodstream as cells called monocytes, then enter the tissues, where they become macrophages. Macrophages consume large foreign particles and cellular debris.

As with red blood cells, the production and number of phagocytes are tightly regulated by chemical messengers in the blood, including interleukins (chemicals found in white blood cells that stimulate them to fight infection). Unlike the red blood cells, which remain circulating in the blood, the phagocytes use the blood’s circulatory system as a pathway to the tissues. Because of this, the number of phagocytes in the blood can provide an indication of disorders in the body. For example, the number of neutrophils increases when inflammation is present anywhere in the body. The number of phagocytes produced as a result of these conditions varies from species to species. An abnormal response, such as an unusually low number of circulating white blood cells 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 "non-self" antigens, such as infectious organisms, foreign tissue, or cancer cells. Lymphocyte production in mammals begins in the bone marrow. Lymphocytes then become T cells, B cells, or natural killer cells. T cells are responsible for a variety of functions, especially fighting off viral infections and cancers. Most T cells remain in the bloodstream, but some are also present in the spleen and lymph nodes. B cells are responsible for producing antibodies that coat invading organisms or foreign substances, marking them for elimination by the immune system. For example, bacteria coated by antibodies are more easily recognized and removed by phagocytes. Natural killer cells and some types of T cells ("cytotoxic" T cells) destroy foreign material, whereas antibodies and other T cells ("helper" T cells) stimulate other components of the immune system to do so. If lymphocytes are decreased or abnormal, the patient is immunodeficient and is at risk for a wide variety of infections.

Antibody molecules are called immunoglobulins. They fall into 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. However, sometimes lymphocytes do not react appropriately. One inappropriate response occurs when antibodies are produced against the body’s own cells. Another inappropriate response of the immune system is allergy. When antibody-primed cells are exposed to an allergen, the reaction can be mild (hives) or life-threatening (anaphylaxis).

An increase in the number of lymphocytes in the bloodstream occurs in some animals as a response to the secretion of epinephrine (a hormone also known as adrenaline). A reduction in the number of circulating lymphocytes may be caused by corticosteroid hormones that are secreted in times of stress. Unusual lymphocytes may occasionally be seen in the blood in response to antigenic stimulation, such as vaccination. Growth of abnormal lymphocytes can result in malignant tumors, such as lymphoma or lymphoid leukemia.

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