What is Antihistamine?
Antihistamine is a class of drugs that inhibit the physiological action of histamines. Histamine is an organic compound, which is involved in local immune responses and also acts as a neurotransmitter. It is also involved in the inflammatory response and is a mediator of pruritus. Histamine is produced by basophils and is found in tissues that are connected nearby. The permeability of white blood cells is increased by histamine and this allows the pathogens to get engaged with the infected tissues.
Action of antihistamine for treatment of allergy
- When our body comes in contact with components such as pollen, dust mites, pet dander etc, a chemical called histamine is formed that triggers an allergy.
- This allergy causes: swelling of the nose, running eyes, and sometimes even itching in the mouth.
- Antihistamines block the action of histamines and protect us from the allergy. They work against the symptoms of different kinds of allergies, which include hay fever, food allergy, etc.
- But their application is limited as they cannot be used to relieve every kind of symptoms.
- If we suffer from a nasal congestion, the doctor will advise an intake of decongestant. Some drugs are formed by the combination of antihistamines and decongestants.
- Antihistamines are used in the treatment of any allergy. Brompheniramine (Dimetapp) and terfenadine (Seldane) are synthetic drugs, which act as antihistamines. These drugs actually interfere with the natural action of histamines. They compete with the histamines in order to obtain the binding sites of the receptor, where histamines exert their effect.
Antihistamines are basically divided into two.
- First generation Antihistamine
H₁ antagonists, also called H₁ blockers, are a class of medications that block the action of histamine at the H₁ receptor, helping to relieve from allergic reactions.
- Second generation Antihistamine
- Generally do not cause the sedation and drying seen in first generation antihistamines
- Do not cross the blood-brain barrier as readily as First Generation compounds
- Large molecular size
- Electrostatic charge
Side effects of antihistamines
Antihistamines are harmful in nature, some are comparatively more harmful than the others.
- The older groups known as the first generation of antihistamines are more harmful, for example Benadryl and Chlor-Trimeton. They cause drowsiness particularly.
- On the other hand, the newer-generation of antihistamine is comparatively less harmful, for example Allegra, Clarinex, and Zyrtec. Dry mouth is the main side effect of antihistamines and their use should be avoided as much as possible.
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