Antihistamines: How do they work?

Antihistamines are medications available in the form of tablets, eye drops, ointments, or nasal sprays primarily used to alleviate allergy symptoms. They can thus be prescribed in cases of allergic rhinitis, characterised by a blocked nose, sneezing, and a runny nose, or urticaria, causing redness and itching. Antihistamines also play a role in the treatment of certain gastric conditions, notably ulcers and gastroesophageal reflux.

Antihistamines aim to inhibit the effects of histamine, a neurotransmitter produced by the body during an allergic reaction.

To better understand the action of antihistamines, it is necessary to first understand histamine and its effects. Histamine is a biological molecule naturally produced by mast cells and basophils when the body feels under attack. This is particularly the case when the skin comes into contact with a substance, object or element that the body perceives as a threat. It can also be a food that the body struggles to recognise and considers as an aggression.

Histamine is thus a component of the inflammatory response that binds to specific receptors, causing various unpleasant symptoms such as coughing, sneezing or itching. Allergic reactions are a way for the body to quickly expel the intruder. This is where antihistamines come into play to block the effects of histamine. The term 'antihistamine' directly corresponds to their primary function. The details of the mechanism of action vary depending on the nature of the antihistamine and the targeted receptor.

• What is the mode of action of H1 antihistamines?

  H1 antihistamines are a class of drugs developed to block H1 histamine receptors, primarily involved in immediate allergic responses, such as seasonal rhinitis, urticaria and certain inflammatory skin reactions. The blocking of these receptors by H1 antihistamines notably allows for the reduction of blood vessel permeability, which is normally increased by histamine. In doing so, these drugs decrease the swelling and redness associated with allergic reactions.

  Furthermore, H1 antihistamines reduce the stimulation of sensory nerves activated by histamine, which helps to alleviate painful sensations and itching. Finally, in the respiratory and digestive tracts, histamine causes smooth muscle contractions, leading to nasal congestion, sneezing and, in some cases, respiratory spasms. H1 antihistamines limit these effects by reducing the muscular response, providing relief from respiratory and skin symptoms.

  Note : The H1 antihistamines are associated with various side effects, such as drowsiness, headaches, and a dry mouth sensation. These vary depending on the generation of antihistamines, with those of the second generation, like loratadine or cetirizine, generally being better tolerated.

• How do H2 antihistamines work?

  Primarily located in the parietal cells of the stomach, H2 receptors play a crucial role in the secretion of gastric acid, a molecule necessary for the smooth functioning of digestion. However, excessive production of acid can lead to gastric ulcers, pain, or gastroesophageal reflux. H2 antihistamines, such as ranitidine and famotidine, block H2 receptors by preventing histamine from attaching to them. This blockage reduces the production of gastric acid, helping to prevent damage to the stomach lining and alleviating symptoms associated with ulcers and reflux. These medications typically act quickly, within a few hours.

• FEINBERG S. The antihistaminic drugs: Pharmacology and therapeutic effects. The American Journal of Medicine (1947).

• SEEVERS M. & al. The toxic effects of anti-histaminic drugs. Journal of the American Medical Association (1951).

• PEARLMAN D. Antihistamines: pharmacology and clinical use. Drugs (1976).