White blood cell

Lore & Background

White blood cells are derived from multipotent hematopoietic stem cells in the bone marrow. They include three main subtypes: granulocytes, lymphocytes, and monocytes. Granulocytes (neutrophils, eosinophils, basophils) are distinguished by lobed nuclei and visible cytoplasmic granules, while agranulocytes (lymphocytes, monocytes) have round nuclei and lack visible granules. Lymphocytes are further divided into T cells, B cells, and natural killer cells, with T cells including helper and cytotoxic subtypes, and B cells including plasma and memory cells. Monocytes can differentiate into dendritic cells and macrophages.

Reader's Guide

White blood cells are essential to the immune system, forming approximately 1% of total blood volume in a healthy adult. An excess (leukocytosis) often signals infection or inflammation, while a deficit (leukopenia) indicates a weakened immune system. Neutrophils are the most abundant, acting as first responders to bacterial and fungal infections, while eosinophils target parasites and are involved in allergic reactions. Basophils release histamine and heparin to mediate allergic responses. Lymphocytes coordinate adaptive immunity through antibody production and cell-mediated responses. The classification of white blood cells by physical characteristics (granulocytes vs. agranulocytes) is less frequently used now, with lineage-based classification (myeloid vs. lymphoid) being more common.

Did You Know?

• The name 'white blood cell' comes from the white layer (buffy coat) seen after centrifugation of blood.
• Neutrophils have a multi-lobed nucleus with three to five lobes connected by slender strands.
• Eosinophils compose about 2–4% of circulating white blood cells and rise in response to allergies and parasitic infections.
• Basophils are the rarest white blood cells, making up less than 0.5% of the total count.