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.
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.
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Skin sensations local itching or burning sensation, flushing and a disagreeable heat.
Stomach pain acid secretion increases; small intestine contracts with crampy pain Respiratory - nose swelling, congestion, sneezing and wheezing.
Anxiety and agitation with diffuse, deep, odd body sensations:"..bones are on fire", "I feel weird all over", "...a deep pricking, crawling sensation...".
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.
These receptors both receive histamine as a messenger, but the meaning conveyed is different.
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.
They also activate the acid-producing, parietal cells of the stomach lining.