The 4 types of antihistamines: characteristics and functions

To this day, allergies are considered a near pandemic picture. The incidence of these autoimmune events 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 Navarra Clinical University (CUN), the allergic reaction consists in the perception of our body as harmful by a substance that is not, known as an “allergen”. When this allergen (latex, mites, animal epithelia, insect bites, food and many more) enters contact with the host, the immune system reacts wrongly, producing a large amount of IgE antibodies.

After the binding of the antibody 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 started, there are a number of drugs and strategies. Next, we tell you

everything you need to know about the types of antihistamines, including times of use.

• Related article: "Types of drugs (according to their use and side effects)"

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, it has the release of this imidazole amine, essential for the expression of immune reactions in the allergic field.

The main task of this inflammatory mediator 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 the processes inflammatory, modulates intestinal contractions at the muscular level and has effects on the heart rate. In other words, it is an autoimmune compound, but it also acts on intestinal motility and is a neurotransmitter of the central nervous system (CNS).

Thanks to her, the leukocytes involved in the allergic immune response can reach the conflict zone earlier and trigger 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 approach of preventing their adherence to structures keys.

• You may be interested in: "Leukocytes: what are they, types and functions in the human body"

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, by blocking its key receptors. Antihistamines are usually consumed massively worldwide, as they are very cheap drugs, in many cases they do not require a prescription (generic) and are effective in alleviating nasal congestion, sneezing and itchy skin, all of which are symptoms associated with allergic processes and autoimmune.

These drugs they act against histamine on two very marked fronts: avoiding 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. Here are the types of antihistamines, based on their pharmacological properties.

1. H1 antihistamines

As his name indicates, they are drugs that inhibit the action of the H1 receptor, which is expressed in smooth muscle, vascular endothelial cells, the heart, and the central nervous system (SNC). It should be noted that there are two generations within this drug block.

First-generation H1 antihistamines are lipophilic and can easily cross the blood-brain barrier (between the blood and the brain). For this reason, they are considered to have a sedative effect, and some drug 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 its side effects do not go through sedation and other processes anticholinergics.

• You may be interested in: "The 13 types of allergies, their characteristics and symptoms"

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 decreases gastric acidity. We remember that histamine is essential for the immune response to an allergen, but by itself it also modulates basic stomach functions (among many other biological elements).

H2 antihistamines, unlike those mentioned, are used to treat peptic ulcers, dyspepsia, stress ulcers, gastroesophageal reflux and other conditions. However, today the use of proton pump inhibitor drugs, especially omeprazole, to address these conditions is more widely conceived.

3. H3 antihistamines

As you can imagine, H3 antihistamines block the histamine H3 receptor, which is expressed mostly in the central nervous system and to some extent in the peripheral. Although it seems similar to the H1 and H2 groups, nothing is further from the truth: the previous ones act at the level of the periphery of the organism (although they cause sedation if they reach the brain environment), while the H3s work directly in the neural and encephalic 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 efflux of neutrophils from the bone marrow and is part of the oral epithelium. There is not much more information about them available to the public.

Side effects and contraindications

If you are here due to an allergic condition, you are interested in knowing the effect and synergy of the H1 antihistamine drugs first and second generation, since the rest are used in other medical fronts, as you have been able to check.

For its part, first-generation H1 antihistamines have a clear sedative effect, as they cross the blood-brain barrier and interact with neurons in the brain. For this reason, it is completely inadvisable to consume alcohol if you are under this drug 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 to say, do not carry out any activity that entails dangers or that requires marked attention. If the patient is in any of these situations, it is much better to turn 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 have 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 related to reducing acidification of the gastric environment. To date, drugs 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 also modulates intestinal motility, gastric acid secretion and even certain circuits at the brain level (being a neurotransmitter).

For all these reasons, antihistamines have a promising future, especially if we look at the H3 family. If given with the right 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.