White blood cell

White blood cells, or leukocytes, are the cellular agents of the immune system, tasked with defending the body against infectious diseases and foreign substances. They are generally larger than red blood cells and are distinguished by the presence of a nucleus, a feature absent in red blood cells and platelets. All leukocytes originate from multipotent hematopoietic stem cells in the bone marrow and are found throughout the body, circulating in the blood and lymphatic system. Their number in the blood is a key health indicator, measured as part of a complete blood count; a healthy adult typically has between 4,000 and 11,000 white blood cells per microliter of blood, making up roughly 1% of total blood volume. An abnormally high count is called leukocytosis, often a normal response to infection or inflammation, though it can occasionally signal blood cancers or bone marrow disorders. A low count, known as leukopenia, indicates a weakened immune system. The name "white blood cell" comes from the appearance of centrifuged blood, where these cells form a thin, white layer called the buffy coat between the red blood cells and plasma; the term leukocyte itself derives from Greek roots meaning "white cell." Historically, leukocytes were classified by physical traits into granulocytes (with visible granules and lobed nuclei) and agranulocytes (with round nuclei), but this system is now less common. Modern classification emphasizes cell lineage: myeloid cells include neutrophils, eosinophils, basophils, and monocytes, while lymphoid cells include T cells, B cells, and natural killer cells. Neutrophils are the most abundant, comprising 60–70% of circulating leukocytes, and are typically the first responders to bacterial or fungal infections, their accumulation forming pus. Monocytes and neutrophils are phagocytic, engulfing pathogens. Lymphocytes are further subdivided into T cells (helper, memory, and cytotoxic types), B cells (plasma and memory cells), and natural killer cells.