Monocytes are the largest white blood cells. Its nucleus is kidney-shaped or horseshoe-shaped. Its diameter is 12-20 µm. Their number is 280 per cubic milliliter. 4% of white blood cells are monocytes. Lifespan 2-5 days. It originates from red bone marrow (monoblast cells), liver, spleen and lymph nodes. 30-40 hours after generation it transforms into macrophage.