White blood cells, or leukocytes, are cells produced by the bone marrow
that are responsible for the body's immune defenses against attacks. They are contained in the blood and the body contains on average between 4000 and 10,000 per mm ?. There are three main types of leukocytes: monocytes
, and neutrophils; there are two types of neutrophils, eosinophils and basophils. A decrease in their number leads to vulnerability to infection and can be due to different types of diseases including those affecting the bone marrow such as leukemias
. An increase may reflect a current action against an infection or other diseases of the bone marrow in particular.