White blood cell

A white blood cell, or leukocyte, is a blood cell that helps defend the body against infections and other foreign substances. Its major types include cells that engulf microbes, produce antibodies, or coordinate immune responses.

Neutrophil: A granulocyte that rapidly engulfs microbes and is commonly recruited to sites of acute infection. Neutrophils are the most abundant circulating leukocytes and prominent first responders to infection.
Leukocyte extravasation: The multistep passage of leukocytes from blood vessels into surrounding tissues. This process brings circulating white blood cells to an infected or injured site.
Complete blood count: A blood test that measures blood cell numbers and reports several cell characteristics. Its white-cell count and differential provide routine measurements of circulating leukocytes.
Red blood cell: A blood cell specialized to transport oxygen and carbon dioxide using hemoglobin. Red blood cells transport gases, unlike leukocytes, whose central role is defense.
Lymphocyte: An immune cell that includes B cells, T cells, and natural killer cells. Lymphocytes provide antibody production, targeted cellular responses, and innate killing.
Chemotaxis: Directed cell movement along a chemical concentration gradient. Chemical gradients guide leukocytes toward signals released at sites of infection.
Leukocytosis: An abnormally high number of white blood cells in the blood. An elevated count can accompany infection, inflammation, medication effects, or blood disorders.
Platelet: A small blood component formed from megakaryocytes that helps stop bleeding. Platelets support clotting rather than serving as the principal cellular defenders against infection.
Monocyte: A circulating white blood cell that can enter tissues and develop into macrophages or dendritic cells. Monocytes connect blood leukocytes to several long-lived immune cells in tissues.
Phagocytosis: The engulfment and internal breakdown of particles by a cell. Neutrophils and monocytes use phagocytosis to ingest microbes and debris.

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