leukocyt

Leukocyte, or white blood cell, is a nucleated cell of the vertebrate immune system. Leukocytes circulate in the bloodstream and lymphatic system and migrate into tissues in response to infection, inflammation, or injury. They protect the body by recognizing foreign invaders, engulfing pathogens, presenting antigens, and coordinating the immune response. Leukocytes differ in size, function, and lifespan.

Leukocytes are broadly categorized into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes and monocytes). Neutrophils are rapid responders that perform phagocytosis; eosinophils combat parasites and modulate inflammation; basophils release mediators of allergic responses. Lymphocytes include B cells, T cells, and natural killer cells and mediate adaptive immunity. Monocytes migrate into tissues and differentiate into macrophages or dendritic cells.

Most leukocytes arise from hematopoietic stem cells in the bone marrow. Development paths are called granulopoiesis, monocytopoiesis, and lymphopoiesis. Lifespans vary widely: neutrophils typically live only a few hours to days in circulation; lymphocytes may persist for years; monocytes survive about one to three days in blood before entering tissues.

Normal leukocyte count ranges roughly 4,000 to 11,000 per microliter, depending on age and lab references. Abnormally high counts are called leukocytosis; low counts are leukopenia. Abnormal counts occur with infection, inflammation, leukemia, immune disorders, and certain medications. The complete blood count (CBC) measures total leukocytes and a differential to estimate the proportions of each type.