White blood cells, also known as leukocytes, are an important part of your body's immune system. Together they circulate in the blood and fight off foreign invaders and infection. At any given time, your body has between 5,000 and 10,000 white blood cells per microliter of blood to help protect the body.
White blood cells are produced in bone marrow, which is located within the bones of the body, more specifically the flat bones such as the pelvis, sternum and skull. There are two types of bone marrow, red and yellow; white blood cells originate from the red bone marrow. Red bone marrow is comprised of myeloid tissue.
Myeloid tissue is an important component of the body because it contains hematopoietic stem cells. Stem cells are the body's building blocks, as they can transform themselves into any cell needed to produce the tissues of the body. As such, stem cells are often referred to as "master cells." Hematopoietic stem cells are unique in that they cause hematopoiesis, or the making of blood. These stem cells, when directed by the body, have the ability to transform into any type of white blood cell the body needs to protect itself from invaders or infection.
The body directs the production of white blood cells so that blood on average contains 7,000 white blood cells per microliter. These blood cells are further divided into six types of white blood cells. More than 50 percent of white blood cells are neutrophils, which protect the body from infection by ingesting (phagocytosis) the foreign invader. The rest of the white blood cell count is made up of a mixture of basophils (which promote inflammation and blood flow to infected areas), lymphocytes (which produce antibodies and attack foreign and tumor cells), monocytes (which eat bacteria and tissue debris) and eosinophils (which react to allergens and parasitic worms).
Although the majority of white blood cells are created after a child's body has developed enough to have bone marrow, other areas of the body also carry the necessary stem cells to produce white blood cells. The spleen, liver and thymus have the ability to produce white blood cells when needed. This process, called extramedullary hematopoiesis, does not usually occur unless the bone marrow has become unable to produce enough white blood cells for the body.