White blood cell

White blood cells, or leukocytes, are the immune cells of the blood. Unlike red blood cells they keep their nucleus and organelles, and they are far fewer: a microliter of blood normally holds roughly 5,000 to 10,000 of them. They are made in the bone marrow and use the bloodstream mainly as a highway. To reach an infection or injury they squeeze out between the cells of a capillary wall, a process called diapedesis, and follow chemical signals to the site.

There are five main types, sorted by whether their cytoplasm has visible granules. The granulocytes are neutrophils, the most numerous, which engulf bacteria; eosinophils, which act against parasitic worms and in allergies; and basophils, which release histamine and heparin. The agranulocytes are lymphocytes, which include the B cells that make antibodies, T cells and natural killer cells, and monocytes, which leave the blood and mature into macrophages that engulf microbes and dead cells.

A white cell count, often with a breakdown of the five types, is part of a routine blood test. A high count, called leukocytosis, often points to infection or inflammation, while a low count, leukopenia, can follow bone marrow damage or HIV infection. Leukemia is a cancer in which abnormal white blood cells are overproduced.