Your nose filters the air you breathe, helping to prevent unwanted substances from traveling into the lungs. As you inhale, the air circulates over, under and around the inferior, middle and superior nasal turbinates (passages). This process filters, moistens and warms the air.

When you breathe in an allergen like pollen, it gets filtered out and lodges in the mucus that lines the nasal passages. There, in a sensitized individual, it encounters mast cells, which react by releasing histamine and other mediators. The histamine causes blood vessels in the nose to dilate and leak fluid into the surrounding tissue. When the fluid seeps into the surrounding tissue, it causes swelling, itching and inflammation. As a result, you experience symptoms like a runny, itchy nose and sneezing.

Mast cells are also found in the mucous membranes lining your eyes (called conjunctiva), so if an allergen gets into your eye, you may have itchy, red, watery eyes. They also are found in your lungs and digestive tract, and in your skin.

Sometimes your allergies may also affect your sinuses, the air-filled cavities lined with mucous membranes in the bones surrounding the nose.