granulocytes

Granulocytes are a category of white blood cells characterized by cytoplasmic granules visible under light microscopy. The principal granulocytes are neutrophils, eosinophils, and basophils. They arise from hematopoietic stem cells in the bone marrow, part of the myeloid lineage, and are released into the bloodstream before migrating to tissues as needed in immune responses.

Morphology and staining: Neutrophils are multilobed with small granules; eosinophils have bilobed nuclei and large red-orange granules; basophils contain abundant granules that stain dark with basic dyes. Their granules carry enzymes and mediators such as myeloperoxidase (neutrophils), major basic protein (eosinophils), and histamine and heparin (basophils).

Functions: Neutrophils are the most abundant granulocytes and act as first responders to bacterial infection, performing phagocytosis and releasing antimicrobial substances. Eosinophils defend against parasites and participate in allergic and inflammatory responses. Basophils contribute to allergic inflammation by releasing histamine and other mediators and can influence other immune cells.

Development and lifespan: Granulopoiesis is driven by cytokines, notably granulocyte colony-stimulating factor (G-CSF). Mature granulocytes are released into blood; neutrophils circulate for about 6–8 hours and live only a few days in tissue, eosinophils for about 8–12 days, and basophils for roughly 1–2 weeks depending on conditions.

Clinical relevance: Abnormal granulocyte counts assist diagnosis. Neutrophilia occurs with bacterial infection and inflammation; neutropenia increases infection risk. Eosinophilia accompanies parasitic infection, allergies, and some drug reactions. Basophilia is rare and may appear in certain myeloproliferative disorders. A complete blood count with differential is commonly used to assess granulocytes.