to produce the early swelling, redness and heat of an inflammatory response.
The same mediators may sensitize nerve endings to other pain-producing mediators such as bradykinin.
An initial burst of mediator activity will often set a series of cell responses in motion which will amplify and prolong disturbances for days or weeks.
The H1 and H2 receptors both receive histamine as a messenger, but the meaning taken by the different receptors is different.
H1 receptors tend to produce the symptoms already listed and activate the allergic reaction.
H2 receptors tend to act as negative feedback receptors and turn the allergic reaction off.
H2 receptors also exclusively activate the acid-producing, parietal cells of the stomach lining.
Antihistamines are drugs which block the receptors so that the histamine messages are not received.
We have drugs that selectively block both kinds of histamine receptors.
The common antihistamines (Benadryl, Chlortripalon, Atarax, Seldane, and Hismanal) are H1 blockers.
Almost everyone has taken an antihistamine to treat hay fever and itching, to relieve nausea and vomiting and cold symptoms, or as an aid to sleep.
The popularity of antihistamines is a mute testimony to the negative effects of histamine.
To get a good idea of what histamine can do, let us imagine the effects of an injection of a small amount of histamine: Histamine carries its message to a large number of cells by attaching to a special receptor on the cells' surfaces.
There are two kinds of histamine receptors, H1 and H2.