When you think of summertime in Britain, certain sounds co: ice clinking, crickets chirping and, of course, sneezing. For hay-fever sufferers this is a bittersweet time of year. However, thanks to molecules like loratadine, or Claritin® to you and I, this time of year is slightly more bearable.

Loratadine is just one of many drugs used to combat hay-fever, which come under the classification of anti-histamine. To fully appreciate the action of loratadine, we need to investigate how histamine mediates an allergic response.

When an allergen, such as pollen, enters the body it binds to antibodies, which have been produced when the allergen was first encountered by cells of the immune system. These antibodies are themselves bound to the surface of mast cells in the tissues and basophils in the blood. These cells contain granules, filled with a group of molecules classified as ‘vasoactive amines’ of which histamine is one. Upon allergen binding to these antibodies, the cells ‘degranulate’ and release their contents.

When histamine is released, it acts on the capillaries, increasing their permeability to white blood cells and proteins. This increased vascular permeability causes fluid to escape from capillaries into tissues, which leads to the classic symptoms of an allergic reaction such as hay-fever: a runny nose and eyes. Histamine has additional clinical effects associated with allergic reactions, including: sneezing due to histamine-associated stimulation of sensory neurons, hyper-secretion from glandular tissue and nasal congestion due to the vascular engorgement caused by vasodilation in the nose.

So, in order to prevent the onset of an allergic reaction, histamine must be stopped from functioning. Loratadine ensures this by acting as an inverse agonist for the receptor histamine binds to. An inverse agonist works by competing with the natural ligand for a receptor and binding, decreasing the level of activity from the basal level.

‘Til next time…


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