Why do we need histamines - If the allergen does not really bother you?

Question

I must begin with the fact that I am not a physician nor a soon to-be one, I ask this question in a naïve and not-so-professional way because I don't want to mislead anyone - the examples and analogies I write here may be false and deceiving - it is because I want to be extra clear and using these examples I will be able to convey the main question/idea.

The big "straight-forward" question is the same as the title:

Why do we need histamines - even if the allergen is not harmless to us?

I will give myself as an explanation to this question. I was diagnosed with Allergic Rhinitis a couple of months ago (even though the allergy was still my "friend" for a long time before that). I am allergic to dust, seasonal allergy and dog/cat fur.

When I do get an allergic attack - My body starts one hell of a ride that consists of: sneezing, fatigue/sleepiness and a runny nose, for at least 10 minutes.

My main question is why shouldn't I take a pill or a medicine that suppresses the "manufacturing" of histamine, if those are the reason I get these kinds of allergic attack. Does sneezing / having a runny nose / being tired actually help the body to treat the allergy? If no, then why do we need histamines to trigger those kind of mechanisms in our body? Would you harm yourself if you would take a medicine that suppresses the manufacturing of histamines?

Comparing myself to other people who are not allergic to (let's say) dog fur, they can pet a dog all day long and not sneeze once, or even wash their hands (let's leave the hygiene behind for a moment). This tells me that the histamines don't trigger an allergic reaction because they don't recognize dog fur as an "enemy" - because it is not (I am not talking about the extreme cases that can lead to death - because this is not the case here - I've pet a lot of dogs and did not die, which means I am not one of these rare cases)!

I hope I was clear, and I also hope I am in the right exchange site.

• I am not asking for any diagnostics, it is a theoretical question.

Thank you very much for your time! :-)

Answer 1

The sneezing and runny nose is certainly a defense of your body against invading foreign bodies, in your case the allergens of pollen, dust mites, dog or cat (usually dander from the skin and/or saliva (Fel d 1 in the case of cats)). In the case of inhaled antigens such as you are experiencing, the runny nose helps wash out the allergen and prevents them physically touching the nasal surfaces. The sneezing physically removes the allergen too. What is going on inside the surfaces of your nose is that the allergen comes in and binds with a type of antibody called Immunoglobulin E (IgE), which then binds to white blood (immune system) cells known as Mast cells and Basophils. These then release histamines to produce the local reaction.

Histamines have a whole bunch of other roles in the body, mediating quite a range of immune functions and there a 4 different types of histamine receptors that do a range of different things in different parts of the body. There's a nice review of these systems at the NCBI bookshelf¹, it is a little technical, but google the terms and you will get some idea of what they are talking about. An even more technical review on Allergy and Allergic Diseases ² can be found at New England Journal of Medicine, but I think it is probably paywalled for public access.

As to why we produce histamines - they are actually part of the immune response to foreign bodies, in particular they help prevent invasion of bacteria by causing local reactions (e.g. swelling) that are the result of an increase in vascular (blood vessel) permeability, which means that the white blood cells of your immune system can more easily approach sites of potential infection (cuts etc.) and prime the immune system to respond more widely to an infection. We can't get rid of histamines, but we can inhibit some of the receptors via chemical means (drugs) such as those used to treat allergies.

In the case of your allergies, the most likely drugs will be Histamine Receptor 1 (H1R) blockers. There are quite a few of these available clinically for a range of different purposes depending on exactly which subtype of receptor they target and how they are used and transported by the body. As these don't block all the types of histamine receptor, and I doubt (but don't know if this is true) that even for the ones they target that they will block all (100%) of the receptors they are targeting, you will still get some histamine functions when taking these drugs. This means that there is no overall impairment of your immune system.

I don't know if it is possible to actually remove (knock-out) histamine production, but I suspect that if you could do so, that it would result in severe pathology with lack of development of brain function, impaired immune response, impaired sleep, body temperature control etc.

1: Patel and Mohiuddin. Biochemistry Histamine 2021. StatPearls Publishing LLC.

2: Kay. Allergy and Allergic Diseases. N Engl J Med 2001; 344:30-37