In the nasal mucus membranes the pollen comes into contact with the mast cells. Recognizing the pollen as an allergen, most cells start producing histamine. The histamine dilates the blood vessels, causing swelling, inflammation and congestion. Because pollen is so fine, it can come in contact with the mucus membranes of the eyes, leading to watery, itchy eyes. Pollen that lands on hair, clothing or pet fur is touched by the allergy sufferer's hands, which then touch the eyes or nose, creating the allergic cascade of symptoms.
Most pollen allergies are respiratory, affecting the nose, eyes, mouth and breathing.
They are usually seasonal. Humidity usually makes the symptoms worse, as the pollen get trapped in the moisture of the air and remains airborne for longer periods of time. The offending allergens are usually pollens, molds, dust mites and animal allergens. The CDC estimates that about 26 million Americans suffer from hay fever each year, with 17.5 million sufferers being 45 or younger. Allergic rhinitis is linked to chronic sinusitis, an inflammation of the facial sinuses, affecting some 35 million Americans.