Merck Manual

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High White Blood Cell Count

(Leukocytosis)

By

Mary Territo

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Jan 2020| Content last modified Jan 2020
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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). The number of white blood cells (white cell count) is normally less than 11,000 cells per microliter of blood (11 × 109 per liter).

The most common cause of an increase in the white blood cell count is

  • The normal response of the body to an infection

Other causes include

  • Certain drugs, such as corticosteroids

  • Cancers of the bone marrow (such as leukemia)

  • Release of immature or abnormal white blood cells from the bone marrow into the blood

There are many types of white blood cell (see Overview of White Blood Cell Disorders), and each type may be increased. However, most commonly, people have an

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