What is a White Blood Cell?

White blood cells, also known as leukocytes, are a key component of the immune system for their role in fighting infections. Derived from the hematopoietic stem cells of bone marrow, white blood cells are named for the milky, nucleated layer where they accumulate after centrifugation from the blood plasma. Although every leukocyte supports the body's immune system response, white blood cells are differentiated by their structure and function into five distinct types: neutrophils, eosinophils, basophils, lymphocytes and monoctyes. (Reference 1)

Neutrophils

Neutrophils are classified as a granulocyte, a white blood cell containing membrane-bound enzyme granules which aid in the digestion of foreign particles. A granulocyte's chief responsibility is to ingest foreign material through endocytosis and eliminate it with its enzyme stores. Measuring approximately 10 microns in diameter, neutrophils have multilobed nuclei and are responsible for targeting and eliminating bacteria and fungi. (Reference 1)
Neutrophils are the most prevalent of all white blood cells and have a lifespan of several days. (Reference 2)

Eosinophils

Eosinophils, also classified as a granulocyte, measure approximately 10 microns in diameter and feature bi-lobed nuclei. Eosinophils target parasitic infections and act as the primary inflammatory cells in the body's allergic reaction mechanism. These white blood cells circulate in the blood for four to five hours and can live up to 12 days. (Reference 1)

Basophils

The last of the granulocytes, basophils are slightly larger than neutrophils and eosinophils with a diameter of 12 microns. Basophils can have a bi- or tri-lobed nucleus and are filled with histamine granules. Basophils function in the allergic and antigen response pathways by releasing histamine to promote blood flow to targeted areas. (Reference 2)
Basophils have a highly variable lifespan and can live from a few hours to a few days. (Reference 1)

Lymphocytes

Lymphocytes are classified as an agranulocyte for their lack of cytoplasmic granules and are commonly found within the lymphatic system. Lymphocytes further differentiate into three lymphocyte derivatives: B cells, T cells and natural killer cells. B cells make antibodies which, by binding to the surface of harmful pathogens, create a targeting system for the rest of the immune cells. B cells also retain "memory" of how to make certain antibodies and have a lifespan of years, providing the immunological basis for vaccines. Helper T Cells coordinate the body's immune response and kill intracellular bacteria, while cytotoxic T cells target and destroy virus-infected cells and tumor cells. Natural killer cells also eliminate infectious targets by releasing the apoptosis-inducing proteins granzyme and perforin. All lymphocytes are around 8 microns and diameter and, like the B cells, can live for years within the body. (Reference 1)

Monocytes

Monocytes are kidney-shaped agranulocytes that travel through the bloodstream and differentiate into tissue-specific macrophages and dendritic cells. Whereas monocytes retain pathogen "memory," the larger macrophages coordinate with T cells to kill pathogens similar to the role of neutrophils. Dendritic cells, the other tissue-specific monocyte derivative, are spindly, multiarmed cells that clean up cell detritus and activate T cells. Monocytes are roughly 15 microns in diameter and live upwards of days whereas macrophages measure 20 microns in diameter and can survive for years. (Reference 1)

References

• Human Anatomy & Physiology; Elaine N. Marieb; 2007
• General Anatomy and Physiology: White Blood Cells

Resources

• National Library of Medicine: WBC Count
• White Blood Cell Percentages