When allergens enter the body through the mucous membranes, they can lead to the release of large amounts of histamine as well as signaling substances such as cytokines and pro-inflammatory leukotrienes in a hypersensitive immune system. In the early stages of an allergic reaction, histamine in particular causes symptoms such as itching, sneezing attacks, fluid accumulation in the tissues, and swelling of the mucous membranes.
Histamine is only problematic in excess
In itself, the tissue hormone has important functions to perform in the body that depend on the histamine receptor in question. However, when it is released in excessive amounts, as is the case with allergic disease, the consequences of histamine can be severe.
If the tissue hormone binds to so-called H1 receptors (H stands for histamine), for example in the lungs and skin, small blood vessels dilate and blood pressure drops. The vessel walls become more permeable and edema forms. The bronchial tubes constrict and intestinal movements increase, lymph flow is increased, and itching occurs.
On the other hand, if histamine docks on H2 receptors of the gastric mucosa‘s vestibular cells, increased gastric acid is released, the stomach becomes acidic, heart rate increases, and pulmonary vessels dilate.
Mode of action of an antihistamine
If the effect of the body’s own tissue hormone histamine is to be reversed or at least weakened, then antihistamines are suitable for this purpose. They are very similar in chemical structure to histamine and block either the H1 or H2 receptors for histamine. Histamine is still released. However, since the binding site is occupied by the antihistamine, it can no longer dock itself.
The reactions of the body already mentioned, such as sneezing and itchy nose, are thus greatly reduced. However, they are not completely gone, since they are also caused by other messenger substances. Antihistamines have an itch-relieving, vasoconstrictor and antispasmodic effect, or they inhibit gastric acid production. Depending on which histamine receptor an antihistamine targets, there are H1 and H2 blockers.
Indications
H1 blockers are suitable for allergic reactions such as hives (urticaria) and other allergic skin conditions, hay fever, drug allergies, and after insect bites. Several generations of antihistamines are now available.
Among the active ingredients of the newer H1 antihistamines is loratadine (duration of action 24 hours). It not only blocks H1 histamine receptors but also reduces the release of histamine and leukotrienes. In addition to oral antihistamines, there are antipruritic preparations (gel, cream, ointment) and eye drops for local application.
H2 blockers (for example, with the active ingredients cimetidine, ranitidine or famotidine) inhibit the production of aggressive gastric acid by the occupant cells in the gastric mucosa and relieve pain. When used to treat gastric ulcers, the ulcers heal more quickly. Oral H1 and H2 blockers are sometimes available over the counter at pharmacies.
Side effects of antihistamines
First-generation H1- antihistamines have the disadvantage that the active ingredients they contain can easily pass the blood–brain barrier and reach the nerve cells in the brain. Therefore, they have a depressant, sedative, and drowsy effect. They are hardly ever used orally anymore.
Representatives of the second generation (for example, with the active ingredients cetirizine, loratadine, ebastine, acrivastine, terfenadine) slow down the activity of the nerve cells only slightly or not at all. Despite a more favorable side effect profile, headaches, gastrointestinal complaints, dry mouth (occasionally) and, in very rare cases, hair loss and/or impairment of liver function may sometimes occur. H2 blockers have basically the same side effects, which are occasional.
