Also known as polymorphonuclear leucocytes (‘polys’), these normally constitute 70 per cent of the white blood cells. They are divided into three groups according to the staining reactions of these granules: neutrophils, which stain with neutral dyes and constitute 65–70 per cent; eosinophils, which stain with acid dyes (e.g. eosin) and constitute 3–4 per cent; and basophils, which stain with basic dyes (e.g. methylene blue) and constitute about 0·5 per cent of the total white blood cells.

constitute 25–30 per cent of the white blood cells. They have a clear, non-granular cytoplasm and a relatively large nucleus which is only slightly indented. They are divided into two groups: small lymphocytes, which are slightly larger than erythrocytes (about 8 micrometres in diameter); and large lymphocytes, which are about 12 micrometres in diameter.

Motile phagocytic cells that circulate in the blood and migrate into the tissues, where they develop into various forms of MACROPHAGE such as tissue macrophages and KUPFFER CELLS.

The granulocytes are formed in the red BONE MARROW, while the lymphocytes are formed predominantly in LYMPHOID TISSUE. There is some controversy as to the site of origin of monocytes: some say they arise from lymphocytes, whilst others contend that they are derived from histiocytes – i.e. the RETICULO-ENDOTHELIAL SYSTEM.

The leucocytes constitute one of the most important of the defence mechanisms against infection. This applies particularly to the neutrophil leucocytes (see LEUCOCYTOSIS). (See also ABSCESS; BLOOD – Composition; INFLAMMATION; PHAGOCYTOSIS; WOUNDS.)