Sinus problems are best thought of as an anatomical problem that interferes with circulation of naturally produced mucus. Usually, the anatomical problem is that the openings of the sinuses themselves are too small for adequate clearance of mucus and other debris. The backed up mucus and debris along with pressure differences from the inside of the sinuses and the outside world result in a variety of symptoms including facial pain and pressure, nasal congestion, and postnasal drip. In addition, patients with chronic sinus problems often complains of ongoing fatigue. Small sinus openings can also result in frequent episodes of acute sinusitis (a sinus infection).
Allergy, on the other hand, is not an anatomical problem. It is simply a reaction from your immune system. Allergies to pollen and seasonal allergens such as pollens and grass are the result of the immune system. When a person has an allergic response to something such as pollen or grass, it is simply an individual’s immune system not recognizing those things as harmless. Instead, our immune system tries to trap those particles which it thinks are harmful by releasing substances into the bloodstream that then cause us to have typical allergy responses such as nasal congestion and itchy watery eyes.