Neutropenia is an abnormally low number of neutrophils in a person's blood. A neutrophil is a type of white blood cell that helps protect the body against infection by destroying bacteria.

Neutropenia may be caused by exposure to radiation, some kinds of infection (such as HIV infection), lack of vitamin B12 or folic acid, an autoimmune disease (such as systemic lupus erythematosus, or SLE), or treatment for certain diseases (especially chemotherapy).