Leukocytes

The number of leukocytes in the blood is often an indicator of disease, and thus the white blood cell count is an important subset of the complete blood count. The normal white cell count is usually between 4 billion/L and 11 billion/L. In the US, this is usually expressed as 4,000 to 11,000 white blood cells per microliter of blood. White blood cells make up approximately 1% of the total blood volume in a healthy adult, making them substantially less numerous than the red blood cells at 40% to 45%. However, this 1% of the blood makes a huge 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, which indicates a weakened immune system.