White blood cells (leukocytes) are an important part of the body's defense against infectious organisms and foreign substances. 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.

Like all blood cells, white blood cells are produced in the bone marrow. They develop from stem (precursor) cells that mature over time into one of the five major types of white blood cells—neutrophils, lymphocytes, monocytes, eosinophils, and 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. The proportion of each of the five major types of white blood cells and the total number of cells of each type can also be determined in a given volume of blood.

Too few or too many white blood cells indicates a disorder. Leukopenia, a decrease in the number of white blood cells to fewer than 4,000 cells per microliter of blood, 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, may result from the normal response of the body to help fight an infection. However, an increase in the number of white blood cells can also result when the regulation of white blood cell development is disrupted and immature or abnormal cells are released into the blood.

Some white blood cell disorders involve only one of the five types of white blood cells. Other disorders may involve a few types together or all five types. Disorders of neutrophils and disorders of lymphocytes are the most common. Disorders that involve monocytes and eosinophils are less common, and disorders involving basophils are rare.