In the allergic reaction, the contact with the substance responsible of the disease stimulates the release of histamine (1H-imidazole-4-ethanamine) a substance present in all the body tissues, particularly in the skin, in lungs and in the mucosa of the digestive apparatus.

The histamine receptors (structures of the nervous system that allow the transmission of a message) can be of 3 types: the H1 receptors (mainly present in the skin, in the bronchial tube, in the stomach and in the gastro-intestinal mucosa), the H2 receptors (mainly present in the stomach) and the H3 (mainly present in brain).

The ANTIHISTAMINE DRUGS, referred to allergies, can block the H1 histamine receptors, so, reducing the histamine release in the blood.

They're used in the cure of of the atopic eczema, of the allergic conjunctivitis and rhinitis, of the acute and chronic urticaria, of some allergic dermatitis and in all the diseases with a high histamine presence in organs and tissues.
They seem to be useless in the treatment of the bronchial asthma and in the symptomatic therapy of the common cold.

These drugs are absorbed mainly in the intestine and in the mouth and its action generally starts after 15-30 minutes, it reaches its maximum effect within 1 hour and ceases after 3-6 hours.
It reduces the reddening, the edema, the itch and the bronchial constriction, of course , only if due to histamine release.

SIDE EFFECTS:
Reported some cases of anorexia, nausea, vomit, constipation, diarrhea, gastric diseases, muscular pain, reduced concentration ability.

Antihistamines are not recommended in patients with prostate hypertrophy or glaucoma and who are already assuming antibiotics or antifungal drugs, due to dangerous interactions.
These side effects are present also in the newest antihistamine drugs, (2nd generation antihistamines).

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