How are allergy symptoms eased with nasal steroids?

Stops inflammation: Steroids (different than the steroids that athletes use) are medicines that have many effects, but one is to turn off inflammatory reactions, especially allergic inflammation. Because they have so many effects, they are problem taken by mouth over time, where they are absorbed and can cause side effects. In the nose they are not absorbed, so there are few side effects.

Blocks mediators: During an allergic reaction in the nose, many mediators are released from mast cells and other cells recruited to cause inflammation including eosinophils. Nasal steroids block the effects of many of these chemical mediators and decrease the number of eosinophils. In contrast, antihistamines only block histamine and not the others (kinins, prostoglandins, paf, ecf, leukotrienes, etc).