17.03.2 Histamine and H1-receptor Antagonists

Histamine has a major role in allergic rhinitis. In the presence of an allergen, the mast cells in the nasal mucosa degranulate to release histamine which actions on the histamine H₁-receptors on the endothelial cell lining of blood vessels to cause contraction of these cells, which increases the space between these cells (Figure 17.6). Fluid and proteins can then move out of the blood vessels to form oedema. This leads to inflammation of the nasal mucosa, which is partly due to the oedema.

Figure 17.6 Histamine and allergy (Copyright Sheila Doggrell, QUT)

The H₁-receptor antagonist fexofenadine is used in the treatment of nasal allergy. It is active after oral administration. Fexofenadine cause suppression of the symptoms in rhinitis and rhinorrhea. It is most effective at the beginning of the hayfever/pollen season when pollen counts are low. When pollen counts get high, there are mediators other than histamine involved in the allergic reaction, and the anti-histamines are ineffective against mediators other than histamine. These newer histamine H₁-receptors antagonists do not get into the central nervous system and therefore cause little or no sedation.