White blood cell counts and white blood cell differentials analyze the number and types of white blood cells present in the body, says WebMD. Both of these tests are part of a complete blood count.
White blood cells defend the body against viruses, bacteria and other agents of infection, according to WebMD. A high white blood cell count is a sign of infection; the body produces more white blood cells when they are necessary to combat pathogens or allergens. Low numbers of white blood cells may indicate immune system suppression or a reaction to medication. Because there are five types of white blood cells that all serve a different function, it is also important for doctors to understand the numbers of each type in relation to one another. This test is a white blood cell differential.
The five types of white blood cells are neutrophils, eosinophils, basophils, monocytes and lymphocytes. The most common type of white blood cells in a healthy person are neutrophils, says About.com. Neutrophils envelop and digest bacteria, destroying themselves in the process. Lymphocytes are the next most abundant white blood cell. Natural killer lymphocytes are responsible for providing general immunity. Monocytes are the largest white blood cells and communicate information to lymphocytes. Eosinophils are especially active against parasites and allergens. The least abundant white blood cells, basophils, are responsible for allergic response.