What are antihistamines for?

Antihistamines are drugs that serve to reduce or eliminate the effects of allergies. To do this, they block the receptors to which histamine binds in allergic reactions.

Histamine, as we will see later, is a substance that is released into the body when an allergic reaction is triggered.

Since the first antihistamines (pyrilamine and diphenhydramine) were discovered in the 1930s and 1940s, hundreds of molecules with antihistamine properties have been developed. What's more, the use of this type of drug has also skyrocketed in recent years.

Also, the introduction of second and third generation antihistamines It has been a great advance in the field of medicine. These drugs are similar in efficacy to first-generation antihistamines, but their adverse effects are much more moderate.

On the other hand, in addition to the treatment of allergies, there are certain antihistamines that can also be used to prevent vomiting, vertigo or to induce sleep in patients with insomnia.

Histamine and its receptors

Many antihistamines are available for intravenous administration.

Histamine is a chemical that participates and is involved in many processes that occur in cells. Among these processes, highlights its role in allergic reactions, inflammation, stomach acid secretion and the transmission of nerve impulses.

In order to intervene in all these processes, it binds to a series of receptors that are distributed throughout the body.

When histamine binds to one of these structures, mechanisms are activated and substances are released that will activate and bind to other structures in order to trigger a physiological action.

Depending on the recipient you join, some actions or others will be triggered. Regarding the receptors to which histamine binds, we can mention the following:

  • Histamine H1 receptorsThese structures are found in the bronchi, in the smooth muscle of the digestive system, and in the brain. When these receptors are activated, there is a construction of both the smooth muscle of the bronchi and the blood vessels.
  • Histamine H2 receptors: H2 receptors are distributed in the mucosa of the stomach, uterus and brain. When activated, it also increases the permeability of blood vessels and the secretion of gastric acid is stimulated.
  • Histamine H3 receptors: these are located in the brain and in the bronchi. They are responsible for cerebral vasodilation and could be involved in a feedback system or feedback negative, whereby histamine inhibits its own synthesis and release from nerve endings.

Classical or first-generation H1 antihistamines

This type of antihistamines are able to easily cross the blood-brain barrier of the brain, which is a kind of membrane that protects the brain. By crossing this barrier and managing to bind to the receptors in this organ, quite intense sedative effects are triggered.

What's more, They also have other properties such as antiemetics and antinetosides. These actions are due to the sedative and anticholinergic effects of these drugs.

Some drugs that are included in this therapeutic group are the following:

  • Ethanolamines: the drug diphenhydramine stands out. Its derivative, dimenhydrinate, has anti-kinetoside properties and is used to prevent travel sickness. In addition, there are studies that this active principle is effective for the treatment of vertigo and in the prophylaxis of vomiting after an operation.
  • Ethylenediamines.

  • Alkylamines.
  • Piperazines.
  • Phenothiazines
  • Piperidines.

Read also: Chlorphenamine: What this antihistamine is and how it works

Second-generation H1 antihistamines

Many patients require tests to determine the degree of allergy to certain substances.
Second-generation H1 antihistamines are drugs that are more selective for H1 receptors that are not found in the brain. as they are not able to cross the blood-brain barrier. The reason for this is that its chemical structure has eliminated the parts that allowed it to cross this barrier easily.

As a result, trigger fewer adverse effects; they produce less sedation and less anticholinergic effects. Some examples of these medications are:

  • Loratadine: it is an antiallergic drug that has almost zero sedative and anticholinergic effects.
  • Ebastine: they do not have anticholinergic or sedative properties, but can cause weight gain. It is useful in treating seasonal allergies.
  • Desloratadine: it is used, above all, in acute cases of allergy.

Read also: Loratadine: what is it for and what are its contraindications?