Your nose has the task of filtering out the unwanted substances and preventing them from entering the lungs. As you inhale, the circulates through the nasal passages, being filtered, moistened and warmed as it does so.

When you inhale an allergen such as pollen or pet dander, it’s filtered out the usual way and becomes lodged in the nasal passages, where it encounters mast cells. If you’re allergic to the substance, the mast cells react by releasing histamine, which causes the blood vessels in the nose to dilute and secrete fluid intio the surrounding tissue. This causes itching, swelling and inflammation, creating symptoms such as an itchy, runny nose and sneezing.

Mast cells are in all the places you experience allergic reactions: in the mucous membranes lining the eyes, in your digestive tract, lungs and skin. They’re also in your sinuses.