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 ). 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 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:
Basophils
Eosinophils
Lymphocytes
Monocytes
Neutrophils
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 × 109 per liter). The proportion of each of the five major types of white blood cells and the total number of cells of each type in a given volume of blood can be determined through laboratory tests.
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 (4 × 109 per liter), 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 (11 × 109 per liter), 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 Overview of Leukemia Leukemias are cancers of white blood cells or of cells that develop into white blood cells. White blood cells develop from stem cells in the bone marrow. Sometimes the development goes awry... read more ) with the release of immature or abnormal white blood cells from the bone marrow into the blood.
Some white blood cell disorders involve only one of the five types of white blood cells.
Neutrophilic leukocytosis Neutrophilic Leukocytosis Neutrophilic leukocytosis is an abnormally high number of neutrophils (a type of white blood cell) in the blood. Neutrophils are a type of white blood cell that help the body fight infections... read more is an abnormally high number of neutrophils
Other disorders may involve a few types together or all five white blood cell types. Disorders of neutrophils and disorders of lymphocytes are the most common. Disorders that involve monocytes Monocyte Disorders Monocytes are a type of white blood cell that fight certain infections and help other white blood cells remove dead or damaged tissues, destroy cancer cells, and regulate immunity against foreign... read more and eosinophils Eosinophilic Disorders Eosinophils are a type of white blood cell that play an important role in the body's response to allergic reactions, asthma, and infection with parasites. These cells have a role in the protective... read more 
are less common, and disorders involving basophils Basophilic Disorders Basophils are a type of white blood cell that have some role in immune surveillance (such as detecting and destroying very early cancers) and wound repair. Basophils can release histamine and... read more are rare.
