We’re sure that many of you have heard about antihistamines, perhaps you are unlucky enough to need them or know someone who is. These medications are usually used to reduce the severity of allergic reactions, along with aid with a few other conditions. Here, we shall discuss how they alleviate the blocked nose, watering eyes and swelling.
Histamine is made from an amino acid called histidine, and stored within mast cells(cells that are found on connective tissue that are part of the immune system). When the body is exposed to an allergen, an immune response is triggered, and antibodies are released to defend the body against what it thinks is a dangerous pathogen.The antibody immunoglobulin E triggers the release of histamine. The histamine’s purpose is to enable more white blood cells to reach the pathogens, and prevent more pathogens from entering the body.
As demonstrated above, the following occur as part of the function of histamine:-
The production of mucus increases to catch the inhaled pathogens.
The secretion of gastric acid increases to destroy anything harmful that is ingested.
Vasodilation(widening of blood vessels) increases the blood pressure in the capillaries, which forces more antibodies into the intercellular space, allowing easier access to the pathogens.
It is clear that histamine is useful in protection against infection, but its effects become detrimental when they are triggered by harmless allergens such as pollen or animal dander. To counter this, antihistamines were created. They have similar structures to histamine, allowing them to act as antagonists- they bind to the same receptors as histamine, preventing them from triggering the reactions.
As can be seen in the diagram below, the role of an agonist, histamine in this case, is to bind to the receptor and produce an effect within the cell. An antagonist, that is the antihistamine, will bind to the same receptor, but not produce a response. As the receptor is not activated but is full, this mechanism blocks the activity of the histamines.
Histamine has many types of receptors such as H1,H2,H3 and H4. For those of you who are interested in knowing the chemical interactions between histamine and its receptors, here’s the fundamentals of the binding process, which can be divided into three steps:
Initial receptor binding : the monocationic form (positive ion, which has lost one electron) of histamine is identified by the H1 receptor. Binding takes place with the help of intermolecular forces such as hydrogen bonding, ion-dipole and ion-ion attractions.
Proton transfer : This is a reversible reaction wherein transferring of proton takes place, which leads to the formation of pros tautomer. Pros tautomer is one of the isomeric(compounds with the same chemical formula but different molecular structure)forms of histamine.
Receptor activation : The intermolecular interactions (such as hydrogen bonding, ion dipole and ion-ion attraction) between the H1 receptor and the pros tautomer allow the receptor to be activated.
As mentioned before, antihistamines bind to the same receptors as histamines. However, they cause the H1 receptors to change to their inactive form, thereby stopping the allergic reactions from occurring.
This brings us to the end of the antihistamine issue, thanks for staying with us till the end. We will be revealing the chemistry behind another interesting topic soon, so stay tuned. Till then, adios!
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Written by:
Mark Monie and Maheshi Parwani
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