azurophilic

Azurophilic is a histological and hematological term used to describe cytoplasmic granules that stain with azure dyes in Romanowsky-type stains. These granules, also called primary or azurophilic granules, are large, electron-dense structures that appear early in myeloid development and are prominent in immature and mature granulocytes, particularly neutrophils and monocytes.

Azurophilic granules contain a mix of antimicrobial substances, including myeloperoxidase, defensins, lactoferrin, lysozyme, and various hydrolases. They contribute to the microbicidal activity of phagocytes, both through enzymatic contents and by releasing reactive molecules during degranulation. The term azurophilic reflects their affinity for azure dyes, which imparts a purple-blue coloration on standard staining.

Development and appearance: Azurophilic granules form during early myeloid differentiation, beginning in promyelocytes before the appearance of specific (secondary) granules. In mature neutrophils, azurophilic granules persist as a component of the cytoplasm, though secondary granules become more numerous as cells mature. Their distinctive staining helps distinguish different stages of granulocyte maturation in cytology.

Clinical relevance: In pathology, azurophilic granules are notable in certain leukemias. Acute promyelocytic leukemia (APL) features promyelocytes rich in azurophilic granules, often containing Auer rods—crystallized, azurophilic, MPO-rich inclusions that are diagnostic for acute myeloid leukemia subtypes, including APL.