ChemCafé

Spring is usually associated with renewal, growth and mating. For some people (including myself) it also means hay fever. So last morning I was taking my daily pill against allergies, I looked at the label on the flask, where it is mentionned that the active ingredient is ‘cetirizine hydrochloride‘, which serves as ‘antihistamine‘. So, what is that? What is histamine, a molecule apparently bad enough to have its associated ‘anti’ compound?

Histamine is a very simple molecule which is present is basically every single cell of our bodies. It is produced through enzymatic decarboxylation of amino acid histidine. Histamine’s many roles include neurotransmission (particularly in the sleep regulation mechanism) and immunological response, explaining why it is involved in various immunological troubles, ranging from relatively mild allergies to severe autoimmune diseases.

Histamine

A high proportion of histamine is stored in cells called mastocytes, which are located mostly at ‘risky’ places where the outside world can come into contact with our internal tissues: skin, lungs, mouth, nose… sounds like places where we can feel allergies right? When an allergy reaction takes place, the (harmless to non-allergic people) allergen interacts at the surface of the mastocyte, inducing the release of a massive amount of histamine in the surrounding environment. This results in well known consequences, such as mucuous secretions, itchiness, conjunctivitis. To produce these effects, histamine needs to interact with particular receptors, called, not so surprisingly, histamine receptors. The easy solution to overcome these effects is to prevent the histamine+receptor interaction: this is done thanks to antihistamine molecules which are also binding to histamine receptors, but without inducing allergic symptoms (pharmacologically speaking, antihistamine is an inverse agonist of histamine).

Finally, what about the widely reported drowsiness side-effect? As stated before, histamine plays a role in the sleep regulation. Histamine metabolism is perturbated upon antihistamine injestion, and one of the side effects is a (slight) inhability to maintain vigilance. Recent drugs (including my relieving cetirizin) are supposed to possess attenuated side-effects, but in my experience it is still not perfect since I tend to feel an urge to sleep after each intake…

Cetirizine, the active compound of many antihistamine drugs.