Leukocytes are stored in the bone marrow, spleen and thymus. Leukocytes are the white blood cells. They are part of the body's immune system and are responsible for fighting infections in the body. They are found in the bloodstream and in the lymphatic system. The number of white blood cells in the body can be an indicator that the body is fighting an infectious disease. As a result, the white blood cell count is an important subset of the total blood cell count.