Histamineplays a major role in the response to allergen exposure. After being released bycells such as the mast cell seen in the previous topic, histamine can bind to several receptors. So far 3 receptors have been identified as shown. Although its action is unclear, the H3 receptor appears to act as a negative feedback receptor.
All histamine receptors activate the same 2nd messenger systems.
Extra info: Histamine 1 uses IP3 and Ca2+ second messengers and Histamine 2 uses cAMP as second messenger. The mechanism of Histamine 3 is unknown at this time.
H1, H2 and H3 receptors are located in each others proximity.
Extra info: Based on the activity of the various receptors, it can be summarized that they are present in different areas. The H1 is present in smooth muscles, vessels, and brain, whereas the H2 is located primarily in the stomach.