White blood cell

Lore & Background

White blood cells are derived from multipotent hematopoietic stem cells in the bone marrow. They are classified by cell lineage (myeloid or lymphoid) or by structure (granulocytes or agranulocytes). The five main types are neutrophils, eosinophils, basophils, lymphocytes, and monocytes. Neutrophils are the most abundant, constituting 60–70% of circulating leukocytes, and are first responders to bacterial or fungal infection, forming pus upon death. Eosinophils compose about 2–4% of white blood cells and primarily deal with parasitic infections and allergic reactions. Basophils are the rarest (less than 0.5%) and release histamine and heparin to aid in allergic and antigen responses.

Reader's Guide

White blood cells are essential to the immune system, defending the body against infections and diseases. Their count in the blood is an important indicator of health: an excess (leukocytosis) often signals infection or inflammation, while a decrease (leukopenia) indicates a weakened immune system. The normal white cell count is between 4 billion/L and 11 billion/L. Although they make up only about 1% of total blood volume, their role is critical—immunity depends on them. The classification of white blood cells has evolved: historically based on physical characteristics (granulocytes vs. agranulocytes), it is now more commonly based on cell lineage (myeloid vs. lymphoid). Myeloid cells include neutrophils, eosinophils, basophils, and monocytes; lymphoid cells include T cells, B cells, and natural killer cells. Monocytes, macrophages, and neutrophils are phagocytic. Lymphocytes include B cells (which produce antibodies) and T cells (helper and cytotoxic). The buffy coat, a thin white layer of nucleated cells seen after centrifugation, gives white blood cells their name. The term 'leukocyte' comes from Greek roots meaning 'white' and 'cell.'

Did You Know?

• White blood cells are generally larger than red blood cells.
• The normal white cell count is usually between 4 billion/L and 11 billion/L.
• Neutrophils are the most abundant white blood cell, constituting 60–70% of circulating leukocytes.
• The buffy coat may sometimes be green if there are large amounts of neutrophils due to the enzyme myeloperoxidase.