Neutropenia occurs when there is a deficiency of neutrophils, a type of white blood cell that is a form of granulocyte. These cells contain neutrally-staining granules, which are small enzyme-filled sacs that assist in killing and digesting microorganisms that the cell engulfs. Mature neutrophils have a segmented nucleus, often referred to as "segs" or "polys," whereas immature neutrophils possess a band-shaped nucleus, known as "bands." Neutrophils typically have a lifespan of approximately three days.Neutropenia can occur during viral infections and as a result of certain treatments, reducing the body's defense against bacterial and fungal infections. Also known as agranulocytosis, it can lead to a condition characterized by frequent, chronic bacterial skin infections, among others. While "agranulocytosis" literally means the absence of granulocytes, there might still be a small number present, a situation referred to as granulocytopenia. This condition can be genetic or develop as a component of other medical issues.Granulocytes include neutrophils, eosinophils, and basophils, named after the staining characteristics of their granules in laboratory settings: neutrophils have mildly staining granules, eosinophils have granules that stain prominently with eosin, an acid dye, and basophils have granules that stain with basic dyes. This classification system dates back to when histochemistry could identify certain structures within cells, though their functions were not yet understood. Despite this, categorizing granulocytes into neutrophils, eosinophils, and basophils remains practical and widely used.Granulocytopenia may specifically refer to a shortage of neutrophils (neutropenia), eosinophils (eosinopenia), and/or basophils (basopenia). Sometimes, "neutropenia" is used interchangeably with granulocytopenia or agranulocytosis.