leucocyte

a white BLOOD cell, so called because it contains no HAEMOGLOBIN . It also differs from red blood cells ( ERYTHROCYTES ) in having a nucleus. Leucocytes are formed in the BONE MARROW , SPLEEN , THYMUS and LYMPH NODES , and there are three types: granulocytes, comprising 70 percent of all white blood cells, LYMPHOCYTES (25 percent) and MONOCYTES (5 percent). Granulocytes help combat bacterial and viral infection and may be involved in allergies. Lymphocytes destroy foreign bodies, either directly or through production of ANTIBODIES , and monocytes ingest BACTERIA and foreign bodies by the process called PHAGOCYTOSIS . In disease, immature forms of leucocytes may appear in the blood (ultimately forming both red and white blood cells).