Neutropenia (noo-troe-PEE-nee-uh) is an abnormally low count of neutrophils, a type of white blood cell that helps fight off infections, particularly those caused by bacteria and fungi.

The threshold for defining neutropenia varies slightly from one medical practice to another. Neutropenia in adults is generally defined as a count of 1,700 or fewer neutrophils per microliter of blood. The cell count indicating neutropenia in children varies with age.

The lower your neutrophil count, the more vulnerable you are to infectious diseases. If you have severe neutropenia — fewer than about 500 cells per microliter of blood — bacteria normally present in your mouth and digestive tract can cause infections.

Jan. 24, 2013