Histamine and antihistamine medications

Histamine and antihistamine: 5 main facts

1. Histamine is one of the main molecules implicated in allergic diseases.
2. Histamine has important roles in the body function, and not only to create allergies.
3. There are several receptors for histamine that trigger different functions in our body.
4. Allergic disease are mostly treated with antihistamines.
5. There are important differences among the different types of antihistamines.

Histamine and antihistamine medications

Introduction

You may have been recently prescribed with antihistamine medications, be curious about this frequently named molecule that seems to be causing your allergy or many other different reasons. Opposite to other websites, I will try to explain the role of histamine on allergic diseases in a way that is simple and easy to understand. Complex technical information can be found elsewhere or asking for a regular consultation in my online allergy consultation. I also offer the service of researching specific topics related with allergic for an hourly price.

What is histamine?

Histamine is a small molecule that was discovered in 1910 and is present in different concentrations in both plants and animal. In humans, it is mainly (but not exclusively) stored in a type of cells called “mast cells”. Mast cells store the histamine in granules, in a way that they can release the histamine in minutes or even seconds, being this the reason why you can present runny nose, sneezing, itch, hives in such a very short time after the exposure to the compound that you are allergic to, or after the stimulus (pressure, heat , cold , vibrations, exercise , alcohol, spices…) that can exacerbate your urticaria.

What is activated in our body by Histamine?

During my online allergy consultation, I have been asked several times about this issue, and the answer usually takes good number of minutes of the consultation although I feel that the answer is rapidly forgotten in the next following weeks due to the complexity of the answer. This is indeed one of the main reasons that lead me to write this article: to help patients to refresh my words in consultation. I will firstly summarize this answer in simple words and later in a deeper and more technical analysis

In summary, histamine promotes itch, swollen skin, airways wall contraction, opening of the blood vessels, and neuronal regulation.

Whenever it is released, a number of receptors are activated in our body, and each of this receptors trigger an specific response. Obviously, several of them can be activated at the same time.

H1 Receptors

Located in the nervous system, blood vessels and smooth muscles. In the brain it helps to control our body temperature, appetite, response to pain, memory and control of the endocrine (hormonal)system. Outside the brain, it promotes the closing of the airways (leading to asthma attacks), the opening of the blood vessels (leading to hypotension and anaphylaxis), the appearance of itch (this is why most allergic diseases are indeed itchy) and other functions like increasing the reactivity of internal organs or contracting of the urinary bladder.

H2 Receptors

Located in the nervous system, blood vessels and smooth muscle, heart, uterus, digestive tube, and some immune cells like mast cell themselves and neutrophils. In the brain, the function is unclear due to the difficulty to block those receptors and see the missing function, but outside the brain it promotes the opening of the blood vessels (leading to hypotension and anaphylaxis), and other functions like contracting of the urinary bladder and increasing the secretion of gastric acids.

H3 Receptors

Located in the nervous system, and likely with multiple functions that has not been already described, it has a complex function that includes autoregulation of the production of histamine and other molecules involved in the function of the neuronal function. Additionally, it regulates the perception of pain, appetite and secretion of gastric acids.

H4 Receptors

Recently discovered in the year 2000, and therefore with many undiscovered functions, it is located in several organs. It regulates itch and the allergic reaction itself, as well as secretion of gastric acids and reactivity of internal organs.

If you are looking for more complex and up-to-date information regarding the role of this receptors in future medications, I recommend this publication from 2018.

What is histamine for?

Mast cells are present in increased numbers especially in those areas that can be exposed to external aggressions (mouth, nose, lungs, internal areas that are in contact with food and air) and blood vessels, and this is why most of the allergic reactions are located in this areas.

Histamine is one of the main molecules in allergic diseases so… why would our body have a molecule that is harmful for us? The answer is that it is not only involved in allergic diseases but it has also many other roles in the normal functioning of our bodies as I will briefly describe here

1.- Immune role:

Allergy is an immune disease, that in an extremely simple way could be described as a “failure” of the immune system that leads to release molecules that can be harmful for us (including histamine) after stimulus that are not really harmful for us, like pollens, molds, dust mites, cat dander, cold, heat, bee and wasp stings, medications, etc.  However, histamine play other roles that are not harmful for us as I will describe now.

Due to the location of mast cells, that are very close to the environment (air, food…) they act are one of the first-line defenses not only as an initial alarm against infections but also as detection for cell stress and tissue damage through a complex system of receptors and signaling.

Thanks to the already stored histamine, they act as one of the first parts of the immune system to work against infections, in a matter of seconds, and this histamine helps to create a swollen skin and opening of blood vessels near the infection area that will help to eliminate the aggressive infectious organism as well as to create a proper environment for other cells to work. The “alarm effect” will create and orchestra of signals and cells recruitment leading to an effective elimination of the infection.

Special mention should be mentioned on the role against parasites, as the swollen mucosa created by histamine effectively disrupts the attachment of parasites to the gut mucosa, and therefore, reduce the number of parasites that can be finally attached to our gut.

2.- Other roles:

Mast cells have a role in protection against cancer, with this role  being a recent focus of attention by researchers, helping the fetus implantation into the uterus, repair of nervous system damage, sexual function, multiple sclerosis, sleep-wake regulation, protection against venoms.

NOTE: Inhibition of histamine with medications may have a number of side effects, but it has not been described an increase or decrease in infections, cancer, pregnancy, repair of nervous system, sexual function, multiple sclerosis, response to venoms. It can, however, disrupt the sleep-wake regulation thanks to the H1 receptor inhibition.

What is the role of Histamine in Allergies?

If the data showed above is summarized, histamine promotes itch, swollen skin, airways walls contraction and opening of the blood vessels.

– Allergic rhinitis: It promotes nasal itch, swollen mucosa and due to the opening of blood vessels, the production of watery secretion thanks to the opening in the walls of the vessels and increased secretion from nasal glans, sneezing due to the activation of the neural system.

– Allergic Asthma: It promotes direct closing of the airways, that is enhanced by the wall thickening caused by the swollen bronchial walls.

– Urticaria/Angioedema: It promotes swollen skin, itch and redness that is no other thing that blood accumulated thanks to the opening of the blood vessels.

– Drug and food allergy: It promotes the full urticarial reaction all over the body, as well as the possible full-body opening of blood vessels that will lead to excess of blood outside of the main arteries and therefore to hypotension and possible risk of death.

Why I am prescribed antihistamines?

If your doctor has prescribed antihistamines, he might have associated your disease with the effects of histamine. You are prescribed antihistamines to reduce the symptoms of your allergic disease. Antihistamines will not avoid the release of histamine by your body, but they will avoid the attachment of histamine to its receptors helping on this way to reduce your symptoms.

Are all antihistamines similar?

The answer is clear: No

There is a wide variety of antihistamines, and your allergist should be aware of the differences among them as well as the proper dosage. Many doctors that are not allergist my always prescribe the same antihistamine regardless of the differences among them, as the knowledge of this differences requires specialization, the same way as allergist may likely be suboptimal for prescription of neurological medications.

If you feel your antihistamine medication is making any side effect (drowsiness, appetite changes, headache….) or simply your allergic disease is not under control, you can ask for consultation in the following link (ask for consultation) or contact me via whatsapp.