rov....good question and I gather that you are excluding anabolic steroids in your question.
2 phases in the allergic reaction.
Phase 1....release of histamine
Phase 2...cellular response by eosinophils.
Phase one is combated by anti-histamines NOT steroids.
Phase 2 is combated by cortico-steroids NOT anti histamines.
Nasal allergies tend to respond better to steroids than antihistamines because of the intense eosinophilic response rather than a histamine reaction.
rov.....this is off the top of my head so I should wait for the response of a chemical pathologist to reply.