What is an antihistamine? How do antihistamines work? They seem to be very prevalent in our society, to the point where everyone appears to be quite comfortable saying the word, and it would seem that most people are at least vaguely aware of what they are. But prevalence and familiarity are no guarantee of knowledge. If it were, then we would all know how to fix our cars, treat common illnesses, and build simple machines. It is perhaps a telling fact about our modern world that the vast majority of people bandy certain terms and concepts about with little or no real understanding of the thing they are talking about. But because their audience is similarly ignorant, and does not want to be discovered as such, no one ever stops to ask what they actually mean. Let’s stop that ridiculous cycle of ignorance right here and have a quick look at these things and how they work.
In medicine, and in pharmacology in particular, there is the concept of agonists and antagonists. The word antagonist is familiar to most people from the world of drama and entertainment, and is generally understood to mean something or someone that works in opposition to the thing under discussion. In terms of dramatic productions, the antagonist, or villain, is the opposite of the protagonist, or hero. An agonist, therefore, is the opposite of an antagonist.
This is not to imply that an agonist does that which an antagonist prevents or mitigates, however. For example, it is not the case that isoprenaline, a sympathomimetic adrenaline agonist, acts upon the human body in the opposite way to alpha and beta blockers, both adrenaline antagonists. Alpha and beta blockers work to inhibit the action of adrenergic receptors, essentially plugging the sites where the chemical adrenaline would normally be taken up into the system. Isoprenaline, being an agonist, does not free up the receptor sites but rather binds to them and increases their activation responses.
Histamine antagonists are therefore that class of pharmaceutical drugs which act to inhibit the action of histamines by blocking the chemical receptors that they need in order to produce any sort of an effect on you. The site blocking that the compounds are able to perform are slightly different in ambition than those generally found in antagonists.
Rather than simply making it physically impossible for histamines to be taken up by the appropriate receptors, the antagonist actually exerts an influence over the receptors as well. In fact, it is more proper to refer to histamine antagonists as inverse agonists for the most part.
An inverse agonist, as the name suggests, actively counter acts the anticipated effect of the thing it is meant to combat. Perhaps it would be helpful to imagine things as being sort of like your ears. If there is conversation going on in your immediate vicinity, you can use a conversation antagonist (a pair of ear plugs) to block the receptors (your ears) and remove the action of the sound on you. Alternatively, you could put in a pair of earphones and turn your music on, and this would be more like an inverse agonist in that you are replacing the effect with one that prevents the conversation from getting into your ears.
Regardless of the biochemistry or psychophysics of the chemical, it is interesting to look at how they actually function in a more general sense. These compounds act primarily to reduce inflammation and itching associated with allergic reactions, insect bites and stings, and other exogenous influences. There is also a class of histamine antagonists which can address endogenous problems like stomach ulcers and acid reflux diseases.
By this point you understand that an antagonist or an inverse agonist would be used to control the overproduction or unwelcome introduction of something in the system. It follows therefore than an antihistamine would be the correct treatment if the problem is excess histamines. So what do histamines do? What is it about them that make us need to take something to get them to calm down?
Histamine is a nitrogen compound that is strongly implicated in many aspects of the mammalian autoimmune response. It is created by basophils and mast cells in your connective tissue, and it is deployed in response to the discovery of pathogens. In a nutshell, histamines increase the ability of white blood cells to reach and destroy foreign pathogens in the blood stream by increasing the permeability of cell membranes.
While this sounds like a wonderfully efficient and convenient way to deal with blood borne diseases, your histamine system can produce far too much sometimes. People who find that they are prone to producing too much in the way of histamines are said to have one or more of a particular class of autoimmune disorders called allergies. An allergy is simply a hypersensitivity to something which is normally considered to be an innocuous substance. The most common allergies involve plant material such as pollen, animal fur or skin, and an intolerance to certain foods such as peanuts or shellfish.
Intolerance to various proteins is the basic hallmark of an allergic reaction. It is the job of the antihistamine to prevent the excessive action of endogenous histamine compounds, and it does so by literally getting between them and their receptors. Once the antagonist has been successful in blocking the receptors, it shows itself to be more of an inverse agonist instead. While the histamines are trying to increase cell permeability and allow the white blood cells unfettered access to whatever pathogen the immune system has detected, the antihistamines are trying to shove that door closed as quickly and as tightly as they can so that the swelling, itching, and other physical symptoms associated with an allergic reaction diminish as quickly as possible.
The action of an antagonist or an inverse agonist is to reduce the effect of that which it is meant to counter. The next time someone asks you what is an antihistamine, you can tell them that it is a particular sort of pharmaceutical which has been specifically designed to block and work in a diametrically opposed way to those immune systems chemicals that help deal with pathogens. They are useful if your own body’s response is a bit too eager, sort of over the top, or if you actually have a physical hypersensitivity to certain known proteins.