Neutropenia -- Not enough neutrophils. A marked decrease in the number of neutrophils, a type of white blood cell filled with microscopic granules that are little sacs containing enzymes, compounds that digest microorganisms.

Neutrophils are a kind of granulocytes. Granulocytes are part of the innate immune system and have somewhat nonspecific, broad-based activity. They do not respond exclusively to specific antigens, as do B-cells and T-cells.

Granulocytopenia or, as it is also called, agranulocytosis results in a syndrome of frequent chronic bacterial infections of the skin, lungs, etc. Although "agranulocytosis" literally means no granulocytes, there may, in fact, be some granulocytes but too few of them, i.e. granulocytopenia. Granulocytopenia can be genetic and inherited or it can be acquired as, for example, an aspect of leukemia.

Neutrophils, eosinophils and basophils are all types of granulocytes. They are named by the staining features of their granules in the laboratory:

• Neutrophils have "neutral" subtle granules;
• Eosinophils have prominent granules that stain readily with the acid dye eosin; and
• Basophils have prominent granules that stain readily basic (nonacidic) dyes.

This classification dates back to a time when certain structures could be identified in cells by histochemistry, but the functions of these intracellular structures were still not yet fathomed. However, the classification of granulocytes into neutrophils, eosinophils and basophils is still widely used (and quite useful).

Granulocytopenia can therefore more specifically involve neutropenia (shortage of neutrophils), eosinopenia and/or basopenia. The term "neutropenia" is sometimes used interchangeably with granulocytopenia or agranulocytosis.