White Blood Cells

White blood cells or leukocytes are cells of the immune system which defend the body against both infectious disease and foreign materials. They are spherical, colorless and nucleated masses involved with host defenses.

Normal white blood cell counts are variable with age and sex. Normal adult range is 4,5000 to 11,000 cells per microliter of blood. Slightly higher counts are seen in children. Elevated counts can be seen in cases of inflammation and infection.

There are several different types of white blood cells. One primary technique to classify them is to look for the presence of granules, which allows the differentiation of cells into the categories granulocytes and agranulocytes.

Types Of White Blood Cells

Granulocytes have differently staining granules in their cytoplasm when viewed under light microscopy. These granules are membrane bound enzymes which primarily act in the digestion of endocytosed particles. There are three types of granulocytes namely neutrophils, basophils and eosinophils.

Agranulocytes are leukocytes characterized by the absence of granules in their cytoplasm. These include lymphocytes, monocytes and macrophages.

Neutrophils (65%) deal with defense against bacteria or fungal infection and other very small inflammatory processes and are usually first responders to microbial infection; their activity and death in large number forms pus.

Eosinophils (4%) primarily deal with parasitic infections and an increase in them may indicate such infection. It has pink cytoplasm.

Basophils (<1%) are chiefly responsible for allergic and antigen response by releasing the chemical histamine causing inflammation. It has blue cytoplasm.

Lymphocytes (25%) are much more common in the lymphatic system. The blood has three types of lymphocytes: B cells, T cells and natural killer cells.

B cells make antibodies that bind to pathogens to enable their destruction. B cells not only make antibodies that bind to pathogens, but after an attack, some B cells will retain the ability to produce an antibody, serve as a memory system.

CD4+ (helper) T cells coordinate the immune response and are important in the defense against intracellular bacteria.  CD8+ (cytotoxic) T cells are able to kill virus infected and tumor cells.

Natural killer cells are able to kill cells of the body which are displaying a signal to kill them, as they have been infected by a virus or have become cancerous.

Monocytes (6%) share the "vacuum cleaner" (phagocytosis) function of neutrophils, but are much longer lived as they have an additional role; they present pieces of pathogens to T cells so that the pathogens may be recognized again and killed, or an antibody response may be mounted. Monocytes are able to develop into the professional phagocytosing macrophage cell after they migrate from the bloodstream into the tissue and undergo differentiation.

Some leukocytes migrate into the tissue of the body to take up a permanent residence at that location rather than remaining in the blood. Such as fixed macrophages in the liver which become known as Kupffer cells. These cells still serve a role in the immune system.