White blood cells, or leukocytes are so named because pus and the buffy coat are white. These spherical cells are the only formed elements with nuclei and other organelles. A healthy person’s WBC count is typically 4,500 to 10,000 per ul of blood. However, the number of a particular type of WBC increases whenever the body encounters pathogens (disease-causing organisms or chemicals) that it destroys.

Like other formed elements, WBCs are derived from the hemocytoblasts in the red bone marrow and their lifespan ranges from a few hours to many years. Their production is regulated by chemical signals released by red bone marrow cells, WBCs, and lymphoid tissues.


White blood cells help provide a defense against pathogens and certain cells either promote or decrease inflammatory responses. Most of the functions of WBCs are performed within tissues located external to blood vessels. WBCs have the ability to move through capillary walls into tissues in response to chemicals released by damaged tissues or pathogens. They are able to follow a “chemical trail” through the tissue spaces to reach the source of the chemical, a behavior called chemotaxis. WBCs move by ameboid movement, a motion characterized by flowing extensions of cytoplasm that pull the cell along. The congregated WBCs then work to destroy dead cells, pathogens, and foreign substances.

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