Allergies are an exaggerated immune response towards certain allergens which are otherwise considered harmless substances.  Many of these allergens can irritate the sinuses and cause sinus congestion.  Symptoms include:  itchy or runny nose, excessive sneezing, post nasal drips, pain in the sinuses due to congestion and coughing.  A recent survey found that 37 million Americans suffer from sinus and allergy symptoms every year.