Neutropenia is an abnormally low number of white blood cells called neutrophils, also known as granulocytes. Decreased numbers of neutrophils result in an increased risk of infection for the affected individuals. Neutropenia is defined as less than 1500 neutrophils per microliter. It is seen most often in people receiving chemotherapy. Its also a rare side effect of many other types of drugs. Some infections also cause neutropenia. When caused by a drug or infection, the neutrophils almost always go back up when the drug is stopped or infection resolves. There are some rare inherited disorders that cause neutropenia.