If allergens enter the body through the mucous membranes, they can lead to the release of large amounts of histamine and signal substances such as cytokines and pro-inflammatory leukotrienes in the case of a hypersensitive immune system. In particular histamine causes in the early phase of an allergic reaction symptoms such as itching, sneezing attacks, fluid retention in the tissue and swelling of the mucous membranes.
Histamine is problematic only in abundance
By itself, the tissue hormone in the body has important functions to perform that depend on the particular histamine receptor. However, if it is released in excessive amounts, as is the case with an allergic disease, the consequences of the 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, then small blood vessels expand and the blood pressure drops. The vascular walls become more permeable, edema forms. The bronchi narrow and the intestinal movements increase, the lymphatic flow is increased, itching occurs.
On the other hand, histamine docks on H2 receptors of the parietal cells of the gastric mucosa, gastric acid is released more, the stomach becomes acidified, the heart rate increases and the pulmonary vessels expand.
Mode of action of an antihistamine
If the effect of the body's own tissue hormone histamine should be abolished or at least mitigated, antihistamines are suitable for this purpose. In their chemical structure, they are very similar to histamine and block either the H1 or H2 receptors for histamine. Histamine will continue to be released. But since the binding site is occupied by the antihistamine, it can no longer dock itself.
The already mentioned reactions of the body such as sneezing and nose itching are greatly reduced. However, they are not completely gone because they are also caused by other messengers. Antihistamines have antipruritic, vaso-sealing and anti-spasmodic properties or inhibit gastric acid production. Depending on which histamine receptor an antihistamine is targeted for, there are H1 and H2 blockers.
H1 blockers are useful in allergic reactions such as hives (urticaria) and other allergic skin conditions, hay fever, drug allergies and insect bites. There are now several generations of antihistamines.
The active ingredients of the newer H1 antihistamines include loratadine (duration of action 24 hours). It not only blocks the 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 topical use.
H2 blockers (for example with the active ingredients cimetidine, ranitidine or famotidine) inhibit the production of aggressive gastric acid by the parietal cells in the gastric mucosa and relieve the pain. In a therapy of gastric ulcers heal the ulcers faster. Oral H1 and H2 blockers are sometimes available over the counter at the pharmacy.
Side effects of antihistamines
The disadvantage of using H1 antihistamines of the first generation is that the active ingredients they contain can cross the blood-brain barrier and reach the nerve cells in the brain. This is why they have a soothing, soothing and drowsy effect. They are hardly used orally anymore.
Representatives of the second generation (for example with the active ingredients cetirizine, loratadine, ebastine, acrivastine, terfenadine) slow the activity of the nerve cells little or not at all. Despite a more favorable side effect profile, headache, gastrointestinal complaints, dry mouth (occasionally) and in very rare cases hair loss and / or impaired liver function may occur. H2 blockers basically have the same side effects that occasionally occur.