total number of white blood cells in the human body
White blood cell The number of leukocytes in the blood is often an indicator of disease, and thus the WBC count is an important subset of the complete blood count. The normal white cell count is usually between 4 × 109/L and 11 × 109/L. In the US this is usually expressed as 4,000 to 11,000 white blood cells per microliter of blood.[3] They make up approximately 1% of the total blood volume in a healthy adult,[4] making them substantially less numerous than the RBCs at 40% to 45%. However, this 1% of the blood makes a large difference to health, because immunity depends on it. An increase in the number of leukocytes over the upper limits is called leukocytosis. It is normal when it is part of healthy immune responses, which happen frequently. It is occasionally abnormal, when it is neoplastic or autoimmune in origin. A decrease below the lower limit is called leukopenia. This indicates a weakened immune system.
Bone marrow Bone marrow is a semi-solid tissue which may be found within the spongy or cancellous portions of bones.[2] In birds and mammals, bone marrow is the primary site of new blood cell production or hematopoiesis.[3] It is composed of hematopoietic cells, marrow adipose tissue, and supportive stromal cells. On average, bone marrow constitutes 4% of the total body mass of humans; in an adult having 65 kilograms of mass (143 lb), bone marrow typically accounts for approximately 2.6 kilograms (5.7 lb).[4] Human marrow produces approximately 500 billion blood cells per day, which join the systemic circulation via permeable vasculature sinusoids within the medullary cavity.[5] All types of hematopoietic cells, including both myeloid and lymphoid lineages, are created in bone marrow; however, lymphoid cells must migrate to other lymphoid organs (e.g. thymus) in order to complete maturation.