White blood cells (leukocytes) are an important part of the body’s defense against infectious organisms and foreign substances (the immune system White blood cells ). 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 medications, such as corticosteroids
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 Overview of White Blood Cell Disorders 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... read more 
), and each type may be increased. However, most commonly, people have an
