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. (For a more complete discussion of White Blood Cells, see Blood Disorders of Dogs: White Blood Cells of Dogs.)

Phagocytes

Phagocytes 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, primarily neutrophils, protect against bacteria and fungi. Others, known as eosinophils and basophils, are involved in allergic reactions. Monocytes become macro-phages in the tissues, and consume large foreign particles and cellular debris.

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. In cats, neutrophils are normally the most numerous type of white blood cell. An abnormally low number of circulating neutrophils due to marrow failure can lower resistance to bacterial infections. Finally, those elements that produce phagocytes may become cancerous, resulting in a disease called myelogenous leukemia.

Lymphocytes

Lymphocytes are white blood cells that produce antibodies against infectious organisms. They also reject foreign tissue and cancer cells. There are 2 types of lymphocytes: T cells and B cells. T cells are responsible for fighting off viral infections and cancer cells. B cells produce antibodies that help destroy foreign invaders, such as viruses, or cells infected by them. Antibodies also can coat bacteria making it easier for phagocytes to consume them. 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 is commonly found in the lungs and intestines; another is the first antibody produced in response to newly recognized foreign microorganisms; a third is the main antibody in the bloodstream; and a fourth 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. This can result in what are called autoimmune diseases (literally, immune diseases directed against the self), such as immune-mediated hemolytic anemia.

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 corticosteroids.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Nemi C. Jain, MVSc, PhD; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM