Definition of Granulocyte

Granulocyte: A type of white blood cell filled with microscopic granules that are little sacs containing enzymes, compounds that digest microorganisms.

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.

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 (non acidic) 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).