They also activate the acid-producing, parietal cells of the stomach lining.

Histamine dilates blood vessels and acts with prostaglandins, PGE2 and PGI2, to produce the early swelling, redness and heat of an inflammatory response.

Skin sensations local itching or burning sensation, flushing and a disagreeable heat.

Almost everyone has taken an antihistamine to treat hay fever, itching, to relieve nausea and vomiting, in tablets and syrups used to treat cough and cold symptoms, or as an aid to sleep.

The popularity of antihistamines is a mute testimony to the diverse negative effects of histamine.

The same mediators may sensitize nerve endings to other pain-producing mediators such as bradykinin.

H1 receptors tend to produce the symptoms already listed and activate allergic reactions.

H2 receptors tend to act as negative feedback receptors and turn the allergic reaction off.

To get a good idea of what histamine can do, let us imagine the effects of an injection of a small amount of histamine: Headache is felt as a pulsating, whole-head pain, often with a sense of pressure.

Fast heart, blood pressure falls, irregular beats with alarming palpitations.

There are two kinds of histamine receptors, H1 and H2.

These receptors both receive histamine as a messenger, but the meaning conveyed is different.