Granulocytes are cells in the innate immune system characterized by the presence of specific granules in their cytoplasm.

[4] Neutrophils are normally found in the bloodstream and are the most abundant type of phagocyte, constituting 60% to 65% of the total circulating white blood cells,[5] and consisting of two subpopulations: neutrophil-killers and neutrophil-cagers.

[7] Once neutrophils have received the appropriate signals, it takes them about thirty minutes to leave the blood and reach the site of an infection.

[10] Neutrophils are professional phagocytes:[11] they are ferocious eaters and rapidly engulf invaders coated with antibodies and complement, as well as damaged cells or cellular debris.

Granule contents of basophils are abundant with histamine, heparin, chondroitin sulfate, peroxidase, platelet-activating factor, and other substances.

[24] When basophils are injured, they will release histamine, which contributes to the inflammatory response that helps fight invading organisms.

Injured basophils and other leukocytes will release another substance called prostaglandins that contributes to an increased blood flow to the site of infection.

Both of these mechanisms allow blood-clotting elements to be delivered to the infected area (this begins the recovery process and blocks the travel of microbes to other parts of the body).

[20] Mast cells are a type of granulocyte that are present in tissues;[3] they mediate host defense against pathogens (e.g., parasites) and allergic reactions, particularly anaphylaxis.

[28] Further research suggests participants receiving therapeutic granulocyte transfusions show no difference in clinical reversal of concurrent infection.