If allergens enter the body through the mucous membranes, they can lead to the release of large amounts of histamine as well as signal substances such as cytokines and inflammatory leukotrienes if the immune system is over-sensitive. In the early phase of an allergic reaction, histamine in particular causes symptoms such as itching, sneezing, fluid accumulation in the tissue and swelling of the mucous membranes.
Histamine is problematic only in abundance
As such, the tissue hormone in the body has important functions that depend on the respective histamine receptor. However, if it is released in excessive amounts, as is the case with an 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, then small blood vessels expand and the blood pressure drops. The walls of the vessels become more permeable, edema forms. The bronchi narrow and the bowel movements increase, the lymph flow is increased, itching occurs.
On the other hand, if histamine docks with H2 receptors in the parietal cells of the gastric mucosa, gastric acid is increasingly released, the stomach becomes acidic, the heart rate rises and the pulmonary vessels expand.
How an antihistamine works
If the effect of the body’s own tissue hormone histamine is to be abolished or at least weakened, then antihistamines are suitable. In terms of their chemical structure, they correspond very strongly to histamine and block either the H1 or H2 receptors for histamine. Histamine continues to be released. However, since the binding site is occupied by the antihistamine, it can no longer dock itself.
This greatly reduces the body’s reactions, such as sneezing and itchy noses. However, they are not completely gone because they are also caused by other messenger substances. Antihistamines have an itch-relieving, vaso-sealing and antispasmodic effect or they inhibit gastric acid production. Depending on which histamine receptor an antihistamine has in its sights, there are H1 and H2 blockers.
H1 blockers are suitable for allergic reactions such as hives ( urticaria ) and other allergic skin diseases, hay fever, drug allergies and after insect bites. There are now several generations of antihistamines.
The active ingredients of the newer H1 antihistamines include loratadine (duration of action 24 hours). Not only does it block the H1 histamine receptors, it 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 use.
H2 blockers (for example with the active ingredients cimetidine, ranitidine or famotidine) inhibit the production of aggressive stomach acid by the parietal cells in the stomach lining and relieve the pain. When gastric ulcers are treated, the ulcers heal faster. Oral H1 and H2 blockers are sometimes available over the counter in the pharmacy.
Side effects of antihistamines
First-generation H1 antihistamines have the disadvantage that the active ingredients they contain pass the blood-brain barrier well and can reach the nerve cells in the brain. Therefore, they have a dampening, calming and sleepy effect. They are rarely used orally.
Representatives of the second generation (for example with the active substances 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 can occur. H2 blockers basically have the same side effects that occasionally occur.