Antihistamines, histamine receptor antagonists or histamine receptor blockers, are drugs used to treat allergic reactions to neutralize the effect of the body’s own histamine. Antihistamines were discovered as early as 1937 and were first used therapeutically in 1942.
What are antihistamines?
Antihistamines are used in allergic immune reactions of the body to neutralize the effects of histamine. Antihistamines are used in allergic immune reactions of the body to reverse the effects of histamine. Histamines bind to receptors to trigger the body’s immune response. Antihistamines block the docking sites of the receptors, of which there are four different types: H1, H2, H3 and H4 receptors. Histamine is a hormone produced by the body and is found in an inactive form primarily in mast cells and leukocytes, which are part of the immune system. If the body is exposed to antigens – foreign, allergy-causing substances – these attach themselves to the leukocytes or to the so-called immunoglobulin E, which is located on the surface of the leukocytes. The leukocytes are destroyed and the histamine stored in them is released. To reduce the consequences of histamine release and to prevent further release of histamines, antihistamines are prescribed and administered by the physician.
Application, effect, and use
Antihistamines are used to treat allergic reactions. Antihistamines not only block the receptors so that histamines cannot bind to them again, it also acts against histamine that has already been released by leukocytes. The receptors are divided into four groups: H1, H2, H3 and H4 receptors. The H1 receptors cause the following reactions in the body: The blood vessels dilate, so that as a result there is a drop in blood pressure. The vessel walls become more permeable. As a result, edema (water retention) occurs in addition to reddening of the skin. While the blood vessels dilate, the H1 receptors in the bronchial tubes have the opposite effect. Asthmatics in particular are at risk, as the bronchial tubes can constrict in a life-threatening manner. Furthermore, the H1 receptors stimulate nerve conduction so that the skin reacts hypersensitively to touch and itching occurs. If the histamines bind to the H2 receptors, this causes reactions in the cardiopulmonary system. The heart rate increases and the pulmonary vessels dilate. Furthermore, they have an inflammatory effect on the gastric mucosa and stimulate gastric acid production, so that gastritis and heartburn can occur. When histamine binds to H3 receptors, self-regulatory processes occur. Histamine release is inhibited. Research on H4 receptors is still in the early stages, but they are thought to have an effect on allergic asthma. Antihistamines cancel the effect of the hormone histamine. Because of this, there are two types of antihistamines: H1 and H2 antihistamines. H1 antihistamines are used mainly for hay fever, urticaria (hives), as well as for other allergic reactions (watery, itchy eyes, runny nose, shortness of breath, etc.). H1 antihistamines have a spasmolytic (antispasmodic) as well as vasoconstrictor effect. The already dilated blood vessels constrict, the permeability of the vascular walls is reduced, so that edema, skin redness as well as itching recede. H2 antihistamines block the H2 receptors so that no inflammatory reactions can be caused in the stomach. H2 antihistamines inhibit the production of stomach acid. Depending on which active ingredient is used, its effect usually sets in between 30 and 60 minutes. Maximum efficacy is reached after about three hours and usually lasts for a day, with the effect steadily diminishing over the course of the hours. In addition to treating allergic reactions, antihistamines are also used to treat peptic ulcers, ADHD, sleep disorders, and Alzheimer’s disease.
Herbal, natural, and pharmaceutical antihistamines.
To date, antihistamines are only available on the market as H1 and H2 antihistamines and are divided into so-called three generations: 1st generation, 2nd generation, and 3rd generation antihistamines.1st generation antihistamines include the following groups of agents: Bamipin, clemastine and dimetindene, promethazine, diphenhydramine, ketotifen and dimenhydriant. These drugs have many side effects. Because of this, they are no longer used in oral form (tablets, etc.). They are mainly used externally with the help of ointments, drops, gels and creams. With the development of 2nd generation antihistamines, the above-mentioned side effects have been reduced or no longer occur. Active ingredient groups of the 2nd generation include azelastine, cetirizine, loratadine, levocabastine, fexofenadine and mizolastine. The dosage forms are tablets, capsules, sustained-release tablets, ointments, nasal sprays, eye drops, and injection or infusion solutions for acute and severe allergic reactions. Some of the antihistamines are available over-the-counter in pharmacies (mainly 2nd generation), but there are also prescription preparations (1st generation) that must be prescribed by a physician. In addition to the chemical-pharmacological products, there are also natural antihistamines that, in combination, can reduce the body’s allergic response. Ascorbic acid, ascorbate and ascorbyl palmitate (vitamin C) ensure that histamine is broken down more quickly. Panthotenic acid (vitamin B5) is an important building block in the production of cortisol in the adrenal glands. Cortisol has anti-inflammatory properties. Calcium and zinc can block the docking sites of the receptors so that histamine cannot attach. Manganese can block the release of histamine and speed its breakdown. Flavonoids are antioxidants that can have an anti-inflammatory effect. The flavonoids hesperidin, rutin, and quercetin may have a stabilizing effect on mast cells, preventing them from being destroyed by antigens and preventing histamine from being released.
Risks and side effects
1st-generation antihistamines have many side effects. H1 antihistamines have good CNS mobility, which means they can cross the blood-brain barrier so they act directly in the brain and spinal cord. As a result, side effects may include fatigue, hypotension, palpitations, headache, nausea, vomiting, and impaired liver and kidney function. Since antihistamines of this group have a sedative (drowsy) effect, the ability to drive and operate machinery is severely impaired. If cardiac arrhythmias, glaucoma, epilepsy, asthma, and liver and kidney dysfunction are present, 1st generation H1 antihistamines must not be taken as they promote these conditions. Antihistamines should not be taken during pregnancy and lactation. 2nd generation antihistamines can no longer penetrate the blood-brain barrier, so that the side effects are considerably reduced. However, the above-mentioned side effects can also occur here, but their occurrence is much rarer. Side effects can also occur with the natural antihistamines. An overdose of vitamins and minerals can result in cardiovascular diseases (including heart attacks) as well as kidney and liver dysfunction.
Interactions with other medications
Antihistamines of the 1st generation may lead to glaucoma formation (glaucoma) in combination with tricyclic antidepressants. Preparations from the azelastine and cetirizine drug groups must not be combined, as cardiovascular disease may result because of the interaction. Antihistamines must not be taken together with analgesics (painkillers), sleeping pills and anesthetics. H1 and H2 antihistamines must not be taken together with beta blockers and ACE inhibitors (drugs against high blood pressure) as well as with blood coagulants (warfarin).
