White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders.All leukocytes are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell.Leukocytes are found throughout the body, including the blood and lymphatic system. Five different and diverse types of leukocytes exist.ll leukocytes are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, including the blood and lymphatic system. Five different and diverse types of leukocytes exist.