White blood cell
Nucleated immune cells that defend against infection and disease.
White blood cells, scientifically termed leukocytes, are the cellular foundation 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 that sets them apart from anucleated 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 residing in the lymphatic system. They are broadly classified by cell lineage into myeloid cells and lymphoid cells, or historically by physical characteristics into granulocytes and agranulocytes, though the latter system is now less common. The myeloid lineage includes neutrophils, eosinophils, basophils, and monocytes, with monocytes further differentiating into dendritic cells and macrophages. The lymphoid lineage comprises T cells (including helper, memory, and cytotoxic subtypes), B cells (which become plasma cells or memory B cells), and natural killer cells. Neutrophils are the most abundant, constituting the majority of circulating leukocytes, and are typically the first responders to bacterial or fungal infections, their accumulation forming pus. Monocytes, macrophages, and neutrophils are phagocytic, actively engulfing pathogens. The number of leukocytes in the blood is a key health indicator, measured as part of a complete blood count. A normal count ranges from 4,000 to 11,000 cells per microliter, making up about 1% of total blood volume. An elevated count, known as leukocytosis, often signals infection or inflammation, but can occasionally indicate blood cancers or bone marrow disorders. A decreased count, called leukopenia, suggests a weakened immune system. The name "white blood cell" comes from the appearance of centrifuged blood, where these cells form a thin, pale buffy coat between red blood cells and plasma; the term leukocyte itself combines Greek roots meaning "white" and "cell."
- field
- Immunology, Hematology
- known_for
- Defending the body against infection and disease; key component of the immune system
- percentage_of_blood_volume
- Approximately 1% in a healthy adult
- main_subtypes
- Granulocytes, lymphocytes, monocytes
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.
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