Cetirizine synthesis pdf2/14/2023 ![]() The action of histamine on H4 receptors induces chemotaxis, cytokine secretion, and upregulation of adhesion molecules. H4 receptors are found on cells of the immune system, in the gastrointestinal tract, in the CNS, and on afferent neurons with primary sensors. H3 receptors are mainly expressed in the CNS and act as an autoreceptor on histaminergic neurons, inhibiting the release of histamine and modulating that of other neurotransmitters. Additionally, histamine stimulates gastric acid secretion via H2 receptors. By acting on H1, histamine causes itching, stimulates secretion from the nasal mucosa, contracts smooth muscle in the bronchi and intestines, and relaxes smooth muscle in small blood vessels. H1 and H2 receptors are distributed in both the peripheral and the central nervous system (CNS) and allow histamine to exert effects on smooth muscles and glands. Histamine can act on four types of receptors: H1, H2, H3, and H4. The concentration of histamine is particularly high in mast cells, that reside in the respiratory tree, gastrointestinal tract, and skin. Histamine is the pivotal mediator of an allergic reaction. ![]()
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