Granulocytes are a category of white blood cells, characterized by the fact that all types have differently staining granules in their cytoplasm on light microscopy. There are three types of granulocytes: neutrophils, basophils, and eosinophils(named according to their staining properties).

Neutrophils are active phagocytes. Neutrophils, which are also known as polymorphonuclear leukocytes (PMN), represent 50 to 60% of the total circulating leukocytes and constitute the ''first line of defense'' against infectious agents or ''nonself'' substances that penetrate the body's physical barriers. Once an inflammatory response is initiated, neutrophils are the first cells to be recruited to sites of infection or injury. Their targets include bacteria, fungi, protozoa, viruses, virally infected cells and tumor cells.

The mature neutrophil has a segmented nucleus as seen in the blue stained cells on capture above (Click for phagocytosis animation).

Basophils are non-phagocytic cells which, when activated, release numerous compounds from the basophilic granules within their cytoplasm. They play a major role in allergic responses, particularly type I hypersensitive reactions.

Capture above: a basophil is located in the center of this picture. It has a lobed nucleus and numerous coarse, dark blue granules in the cytoplasm.

Eosinophils are responsible for combating infection by parasites in the body. Eosinophils play a role in fighting viral infections; in the allergic response, and in fibrin removal in inflammation. Eosinophils are considered the main effector cells in asthmapathogenesis and are associated with disease severity.

An eosinophils has a nucleus that usually has two lobes connected by a slender thread cytoplasm containing coarse, round granules that are uniform in size.