I've found this really interesting.

Allergic Rhinitis has been shown to be a defining precursor of Asthma. Who knew? The basic mechanism around allergic Rhinitis is the inflammation of nasal passages and sinuses. Due to an allergen the body reacts by releasing cells called Mast cells. These Mast cells usually line mucous membranes and tend to have Histamine particles within their cellular structure. Once released, they excrete the Histamine into the bloodstream which in turn causes the allergic reaction that you see.

Asthma works in the same way: a reaction that inflames the upper respiratory tracts.

Now you know!