Leukocytes (white blood cells, WBCs) constitute the body's primary defense against "foreignness"; that is, leukocytes protect the body from foreign organisms, substances, and tissues. The main types of leukocytes are neutrophils, monocytes, eosinophils, basophils, and lymphocytes. All of these cells are produced in the bone marrow. However, lymphocytes may be produced in additional sites. Each of these types of leukocytes has different functions, and each behaves as a related but different system.34
Neutrophils and monocytes, the most mobile and active phagocytic leukocytes, are capable of breaking down various proteins and lipids such as those in bacterial cell membranes. The function of eosinophils is uncertain, although they are believed to detoxify foreign proteins that enter the body through the lungs or intestinal tract. The function of basophils also is not clearly understood, but the cells themselves are known to contain heparin, histamine, and serotonin. Basophils are believed to cause increased blood flow to injured tissues while preventing excessive intravascular clotting. Lymphocytes play an important role in immunity and may be divided into two main categories, B lymphocytes and T lymphocytes. B lymphocytes are responsible for humoral immunity and antibody production. It is B lymphocytes that ultimately develop into the antibody-producing plasma cells (see Fig. 1-2). T lymphocytes are responsible for cellular immunity and they interact directly with the antigen.35,36 Lymphocytes and related studies are discussed in greater detail in Chapter 3 - Immunology and Immunologic Testing.
Note that leukocytes perform their functions outside the vascular bed. Thus, WBCs are merely in transit while in the blood. Because of the many leukocyte functions, alterations in the number and types of cells may be indicative of numerous pathophysiologic problems.