Overview of white blood cell disorders

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White blood cells (leukocytes) are an important part of the body’s defense against infectious organisms and foreign substances (the immune system). To defend the body adequately, a sufficient number of white blood cells must receive a message that an infectious organism or foreign substance has invaded the body, get to where they are needed, and then kill and digest the harmful organism or substance (see figure Lymphatic System: Helping Defend Against Infection).
Like all blood cells, white blood cells are produced primarily in the bone marrow. They develop from stem (precursor) cells that mature into one of the five major types of white blood cells:

Neutrophils
Lymphocytes
Monocytes
Eosinophils
Basophils

Normally, people produce about 100 billion white blood cells a day. The number of white blood cells in a given volume of blood is expressed as cells per microliter of blood. The total white blood cell count normally ranges between 4,000 and 11,000 cells per microliter (4 to 11