Is it true that allergies block your nose and make your sinus' drain in the back of your throat everyday? what could be done to stop this?

Not exactly.: Allergies typically cause itching, watering, redness. May be associated swelling and "stuffiness" doesn't sound like you have a lot of these symptoms. In any case allergy symptoms will at least partially respond to otc antiallergy meds like loratadine. It may be that you have vasomotor rhinitis which may respond to otc decongestants. Use long acting kind -12 to 24 hour- over the counter.

Nasal Congestion : Yes, allergies cause congestion, which leads to obstruction of the sinuses over time. Subacute sinusitis is common. It can be hard to tell when you have only allergies vs when you have progressed to sinusitis. Antihistamines should not be used if your throat is sore from drainage, because they lengthen the time needed to clear infections. Use nasal steroid sprays, mucinex, (guaifenesin) decongestants, irrigate.

Yes that can be true: But if you see an allergist, you can find out exactly what's causing your symptoms. Then there are many things you can do to avoid those items.. Or at least minimize your exposure to them. Also your allergist can prescribe nasal sprays to minimize or prevent the nasal congestion. This will help prevent or treat any sinus symptoms and postnasal drip.