Eosinophil granulocytes, usually called eosinophils (or, less commonly, acidophils), are white blood cells that are one of the immune system components responsible for combating infection and parasites in vertebrates. Along with mast cells, they also control mechanisms associated with allergy and asthma. They are granulocytes that develop during Haematopoiesis in the bone marrow before migrating into blood.These cells are eosinophilic or 'acid-loving': Normally transparent, they appear brick-red after staining with eosin, a dye, using the Romanowsky method. The staining is concentrated in small granules within the cellular cytoplasm, which contain many chemical mediators, such as histamine and proteins such as eosinophil peroxidase, ribonuclease (RNase), deoxyribonucleases, lipase, plasminogen, and major basic protein. These mediators are released by a process called degranulation following activation of the eosinophil, and are toxic to both parasite and host tissues.
In normal individuals eosinophils make up about 1-6% of white blood cells, and are about 12-17 micrometers in size. They are found in the medulla and the junction between the cortex and medulla of the thymus, and, in the lower gastrointestinal tract, ovary, uterus, spleen, and lymph nodes, but not in the lung, skin, esophagus, or some other internal organs under normal conditions. The presence of eosinophils in these latter organs is associated with disease. Eosinophils persist in the circulation for 8-12 hours, and can survive in tissue for an additional 8-12 days in the absence of stimulation.
Development, migration and activation :
Eosinophils develop and mature in bone marrow.
They differentiate from myeloid precursor cells in response to the cytokines interleukin 3 (IL-3), interleukin 5 (IL-5), and granulocyte macrophage colony-stimulating factor (GM-CSF). Eosinophils produce and store many secondary granule proteins prior to their exit from the bone marrow. After maturation, eosinophils circulate in blood and migrate to inflammatory sites in tissues, or to sites of helminth infection in response to chemokines like CCL11 (eotaxin-1), CCL24 (eotaxin-2), CCL5 (RANTES), and certain leukotrienes like leukotriene B4 (LTB4). At these infectious sites, eosinophils are activated by Type 2 cytokines released from a specific subset of helper T cells (Th2); IL-5, GM-CSF, and IL-3 are important for eosinophil activation as well as maturation.
br> Functions :
Following activation, eosinophils effector functions include production of :
- cationic granule proteins and their release by degranulation.
- reactive oxygen species such as superoxide
- lipid mediators like the eicosanoids from the leukotriene (e.g., LTC4, LTD4, LTE4) and prostaglandin (e.g., PGE2) families.
- enzymes, such as elastase.
- growth factors such as TGF beta, VEGF, and PDGF.
- cytokines such as IL-1, IL-2, IL-4, IL-5, IL-6, IL-8, IL-13, and TNF alpha.
Eosinophil granule proteins :
Following activation by an immune stimulus, eosinophils degranulate to release an array of cytotoxic granule cationic proteins that are capable of inducing tissue damage and dysfunction. These include: