Leukopenia, a decrease in the number of white blood cells to fewer than 4,000 cells per microliter of blood, frequently makes people more susceptible to infections.
Leukocytosis, an increase in the number of white blood cells to more than 11,000 cells per microliter of blood, is often caused by the normal response of the body to help fight an infection, or to some drugs such as corticosteroids. However, an increase in the number of white blood cells is also caused by cancers of the bone marrow (such as leukemia) or by the release of immature or abnormal white blood cells from the bone marrow into the blood.