Granulocytes are a type of leukocyte, or white blood cell. These cells are part of the immune system, and are involved in several different types of immune reaction. There are three types of granulocytes: neutrophils, basophils, and eosinophils.
All three types of granulocyte develop and differentiate in the marrow of long bones such as the femur. All granulocytes start as myeloblasts, immature cells type which has the potential to differentiate into any type of granulocytic cell. Each cell progresses through several stages before emerging from the bone marrow as a mature granulocyte. In the blood, mature neutrophils account for around 55% of white blood cells, while 3% to 6% are eosinophils, and less than 0.5% are basophils. The proportions of these cell types in the blood are fairly constant; measuring blood granulocyte count is therefore a useful tool in diagnosing many types of disease.
As mature white blood cells, granulocytes circulate in the blood until they receive chemical signals that provide further instructions. These signals come from sites of infection or inflammation, and may be chemicals secreted by pathogens or by other cells of the immune system. Each cell type responds to slightly different chemical signals.
Neutrophils respond to chemicals produced by injured cells during inflammatory reactions. Inflammation is often caused by pathogenic infections, but may also be the result of physical trauma caused by cold, heat, stress, or injury. For example, cells injured as a result of an ankle sprain release chemicals that attract neutrophils to the site of injury.
Eosinophils respond to chemicals produced by parasites which invade the body. In addition, these cells are active during allergic reactions. Basophils are involved in allergic reactions as well as immune responses to invading pathogens. Both of these cell types operate in similar fashion to neutrophils, migrating from the blood to tissues in response to chemical signals.
Once the cells reach the site of inflammation, they have multiple roles to play in the immune response. All three of these cell types are phagocytes, and ingest cellular debris, including pathogens and dead or dying tissue cells. Once ingested, these particles are broken down by toxic chemicals which are stored by the granulocytes in membrane-bound granules.
In addition, each cell type can release chemicals into the extracellular environment. Some chemicals contribute to the inflammatory response by stimulating other immune cells, and others are toxic to pathogens. Some of the chemicals released by eosinophils and basophils, such as histamines, are involved in allergic reactions.