The 4 Types Of Antihistamines: Characteristics And Functions
Today, allergies are considered an almost pandemic condition. The incidence of these events of an autoimmune nature is on the rise, since it is estimated that, Today, there are about 20 million people with allergies, of which at least 10 million have symptoms that recur annually. The outlook is not very encouraging, since the World Health Organization believes that, by the year 2050, 50% of the population will suffer from an allergy.
According to the Universidad Clínica Navarra (CUN), an allergic reaction consists of our body’s perception of a substance that is not harmful, known as an “allergen.” When this allergen (latex, mites, animal epithelia, insect bites, foods and many more) comes into contact with the host, the immune system reacts incorrectly, producing a large amount of IgE antibodies.
After the antibody binds to the allergen, there is a release of mediators (including histamine), which cause the typical symptoms of an allergic condition. To prevent the condition from getting worse once the immune system has been activated, there are a series of drugs and strategies. Next, we tell you everything you need to know about the types of antihistamines including moments of use.
What is the role of histamine in allergic reactions?
Before dissecting what an antihistamine is, it is necessary to explore the role of histamine in allergic reactions. Once the IgE-antibody (allergen) binding has occurred, this imidazole amine is released, essential for the expression of immune reactions in the allergic area
This inflammatory mediator’s main job is to cause arteriolar dilation (hypotension), which in turn considerably increases vascular permeability. It also stimulates the secretion of gastric acids, plays an essential role in inflammatory processes, modulates intestinal contractions at the muscular level and has effects on heart rate. In other words, it is a compound of autoimmune nature, but it also acts on intestinal motility and is a neurotransmitter of the central nervous system (CNS).
Thanks to her, leukocytes involved in the allergic immune response can reach the conflict zone earlier and initiate the local or systemic reaction Due to its activity, histamine is one of the main mediators in the allergic response, and that is why antihistamines arise with the focus of preventing its adhesion to key structures.
What are antihistamines and what types are there?
As the Oxford Languages dictionary indicates, an antihistamine is any substance that combats the effects of histamine in the body, through blocking its key receptors. Antihistamines are usually consumed massively worldwide, as they are very cheap, in many cases they do not require a prescription (generic) and are effective in relieving nasal congestion, sneezing and itchy skin, all symptoms associated with allergic and autoimmune processes.
These drugs They act against histamine on two very marked fronts: preventing local inflammation (wheal response) and vasodilation (flare response), by blocking histamine receptors or reducing the activity of its receptors at the nervous, vascular, muscular and endothelial levels. Below, we present the types of antihistamines, based on their pharmacological properties.
1. H1 Antihistamines
As their name indicates, they are drugs that They inhibit the action of the H1 receptor, which is expressed in smooth muscle, vascular endothelial cells, the heart and the central nervous system (CNS). It should be noted that two generations are distinguished within this medicinal block.
First-generation H1 antihistamines are lipophilic and can easily cross the blood-brain barrier (between blood and brain). For this reason, they are considered to have a sedative effect, and some pharmacological variants are even used to treat insomnia problems. On the other hand, second-generation H1 antihistamines are more specific and do not cross this barrier, so their side effects do not involve sedation and other anticholinergic processes.
2. H2 Antihistamines
H2 antihistamines are drugs that block the action of histamine at the level of H2 receptors, this time in the parietal cells of the stomach, which reduces gastric acidity We remember that histamine is essential for the immune response to an allergen, but on its own it also modulates basic stomach functions (among many other biological elements).
H2 antihistamines, unlike those mentioned, They are used to treat peptic ulcers, dyspepsia, stress ulcers, gastroesophageal reflux and other conditions In any case, today the use of proton pump inhibitor drugs, especially omeprazole, is more widely conceived to address these conditions.
3. H3 Antihistamines
As you can imagine, H3 antihistamines block the histamine H3 receptor, which is expressed primarily in the central nervous system and to a certain extent in the peripheral one. Although it may seem similar to groups H1 and H2, nothing could be further from the truth: the previous ones act at the periphery of the organism (although they cause sedation if they reach the brain environment), while H3 work directly in the neuronal and brain environment. .
Thus, Beyond allergies, group 3 antihistamines are beginning to be used in neurological problems, such as narcolepsy, Alzheimer’s, attention deficit hyperactivity disorder (ADHD) and schizophrenia. In any case, at this point medicine continues to investigate the properties of these drugs at the brain level.
4. H4 Antihistamines
H4 antihistamines inhibit the activity of the H4 receptor, which appears to mediate the release of neutrophils from the bone marrow and is part of the oral epithelium There isn’t much more information about them available to the public.
Side effects and contraindications
If you are here because of an allergic condition, you are interested in knowing the effect and synergy of the first and second generation H1 antihistamine drugs, since the rest are used on other medical fronts, as you have seen.
For its part, First-generation H1 antihistamines have a clear sedative effect, since they cross the blood-brain barrier and interact with brain neurons For this reason, it is completely inadvisable to consume alcohol if you are under this pharmacological treatment: the depressant effects of both compounds together can be very dangerous.
Likewise, when faced with a first generation H1 antihistamine, It is recommended not to take the car or operate heavy machinery, that is, not carrying out any activity that entails dangers or requires marked attention. If the patient is in any of these situations, it is much better to resort to second generation H1 antihistamines. We remember that these do not cross the blood-brain barrier and, therefore, do not cause such marked symptoms in the central nervous system (CNS).
Anyway, The second generation also has certain drawbacks: Among its side effects, headaches, cough, fatigue, sore throat and abdominal pain are reported. They are not very common reports in general society, but it is necessary to expose them.
On the other hand, it should be noted that H2 antihistamines are usually well tolerated by the general population, but we remember that their use is about reducing the acidification of the gastric environment. To date, medications such as omeprazole have been shown to be more effective (and with fewer adverse effects) than these drugs.
Summary
As you have seen, the world of antihistamines goes far beyond avoiding allergies. Histamine is a compound that plays an essential role in the immune reaction to the presentation of an allergen, but it also modulates intestinal motility, the secretion of gastric acids and even certain circuits at the brain level (as it is a neurotransmitter).
For all these reasons, antihistamines have a promising future, especially if we look at the H3 family. If given the appropriate guidelines and modifications, these could become the answer to the treatment of diseases such as Alzheimer’s or disorders such as ADHD. Only time and science will tell.
