There is a wide range of normal immune functions and a wide range of normal numbers of circulating immune cells. For example, the normal range of leukocytes in the blood of humans is 5000 to 10,000/mm'. Within this range there are no discemible differences in susceptibility to infection. In fact, there is little increased risk of infection unless total white blood cells drop below 1000/mm Functional terms used to describe the immune system include natural (innate) and specific (acquired). Natural immunity is a result of molecules such as cytokines