Allergy-Immunology glossary

ABSTRACT

Mast cells are normally present in small numbers in the connective tissue of all organs, but particularly in the dermis. They have been considered the tissue equivalent of basophils but there is evidence that both arise from common precursor cell in the bone marrow. The two cell types are readily distinguished by their morphology on light microscopy and the presence of chloroacetate esterase activity in mast cells. Mast cells are specifically involved in type I hypersensitivity reactions where they release their granule contents of histamine and heparin, and synthesize and secrete other mediators into the surrounding tissues. Although serotonin is not normally present, it has been demonstrated in mast cells in the stronia of carcinoid tumours and in mastocytosis[1]. Mast cells are also involved in delayed hypersensitivity, cytotoxicity, immunoregulation and inflammation[2]. Recently, mast cells were reported to be essential in regulatory T-cell-dependent peripheral tolerance[3]. It is a biogenic monoamine found in plant and animal tissue and is released from mast cells as part of an allergic reaction in humans. It stimulates gastric secretion and causes dilation of capillaries, constriction of bronchial smooth muscles, and decreased blood pressure[5]. Histamine, like many other transmitters, mediates responses via receptors [Rs], which are divided into three subtypes H1, H2 and H1. H1Rs are found in the smooth muscle of the intestines, bronchi, and blood vessels. The H2Rs are found in gastric parietal cells and in the vascular and central nervous systems. H3Rs are found in the brain and in the periphery and regulate histamine release[5]. Most recently, a novel orphan G-protein coupled receptor, named H4R [GPRv53, human 390 aa] has been cloned and characterized. It is most closely related to H3R. Unlike H3R, H4R has a distinct tissue distribution and it is localized in the peripheral blood leukocytes, spleen, thymus and colon[6]