Any of several kinds of colorless or nearly colorless cells of the immune system that circulate in the blood and lymph. Leukocytes comprise granulocytes and agranulocytes.
Neutrophils, 55% to 70% of all leukocytes, are the most numerous phagocytic cells and are a primary effector cell in inflammation. Eosinophils, 1% to 3% of total leukocytes, destroy parasites and are involved in allergic reactions. Basophils, less than 1% of all leukocytes, contain granules of histamine and heparin and are part of the inflammatory response to injury. Monocytes, 3% to 8% of all leukocytes, become macrophages and phagocytize pathogens and damaged cells, esp. in the tissue fluid. Lymphocytes, 20% to 35% of all leukocytes, have several functions: recognizing foreign antigens, producing antibodies, suppressing the immune response to prevent excess tissue damage, and becoming memory cells.
Leukocytes are formed from the undifferentiated stem cells that give rise to all blood cells. Those in the red bone marrow may become any of the five kinds of leukocytes. Those in the spleen and lymph nodes may become lymphocytes or monocytes. Those in the thymus become lymphocytes called T lymphocytes.