People suffering from allergies are probably familiar with what antihistamines are , because it is these drugs that can inhibit the action of the main inflammatory mediator – histamine. Histamine is a biologically active substance that is produced in the body of every person, but under normal conditions, that is, with strong immunity and good health, it does not cause an inflammatory reaction, as it is in a bound state. 

The physiological effect of histamine in the body

Histamine acts on the following systems:

· Muscular – causes spasm of smooth muscles

Vascular – relaxes the walls of blood vessels, resulting in a decrease in blood pressure

Digestive – promotes the secretion of gastric juice

· Central nervous system – has the ability to excite the central nervous system. This effect of histamine is associated with the presence of receptors in the nervous system that respond to this substance.

In total, the body has 3 types of receptors with which histamine binds:

  • H1 receptors – located in the smooth muscles (most of all in the bronchi and gastrointestinal tract)
  • H2 receptors – located in the glands of external secretion (contribute to the secretion of gastric juice)
  • H3 receptors – located in the central nervous system

When histamine passes from a bound state to a free state, its physiological effect on the body intensifies. In such cases, antihistamines are used that suppress this undesirable effect. 

The effect of taking antihistamines

Depending on the localization of the action of histamine, 3 groups of histamine blockers are distinguished:

1. H1-blockers are substances that are prescribed to suppress an allergic reaction of an immediate type. Such painful conditions include anaphylactic shock, Quincke’s edema, urticaria. In addition, drugs of this type are prescribed for bronchial asthma, shock conditions (frostbite, burns).

2. H2-blockers affect the parietal cells of the stomach, that is, reduce their activity. As a result of the action of H2-blockers, there is a decrease in increased secretion of gastric juice. Most often, antihistamines with a similar effect are prescribed for gastrointestinal diseases – gastritis, peptic ulcer.

3. H3-blockers are drugs that affect the central and peripheral nervous system. They contribute to the inhibition of nervous processes. Antihistamines of this group are prescribed for neurological diseases, hysteria, convulsions.

How antihistamines work is not fully understood. It is believed that these drugs activate an enzyme that breaks down histamine (histaminase), thereby displacing free histamine from target cells. In addition, some scientists claim that the mechanism of action of drugs is based on the blocking of histamine receptors. In this case, mast cells secrete histamine, but smooth muscles, capillaries and endocrine glands do not respond to its effect. 

In addition to the main action, histamine blockers have a calming effect, relieve inflammation, lower body temperature, nausea and vomiting, and have a weak analgesic effect. The most common form of drug release is tablets and ampoules for injection.

Medicines of the indicated action have practically no side effects, although in some people they can cause dizziness and a decrease in immunity (a decrease in white blood cells in a general blood test). If the patient takes drugs that affect the nervous system, for example, diphenhydramine, he needs to give up hard mental and physical work for a while.

event_note April 9, 2020

account_box Winona Tse MD

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