White blood cells, or leukocytes (also spelled "leucocytes"; from the Greek word leuko- meaning "white"), are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. They live inbetween 6 hours and two days. Leukocytes are found throughout the body, including the blood and lymphatic system.