The substance causing the symptoms is called an allergen. Because the offending allergen is usually a specific protein, people may be allergic to only one substance, or to several. According to the American Medical Association, about 40-50 million Americans are affected by allergies. Outdoor Allergies Pollen . Certain trees, grasses and weeds reproduce by producing a fine, powdery substance called pollen.
Air carries the pollen from the male structures to the female ones.
Because it is airborne, pollen can enter the human body in several ways. As air is breathed in through the nose, nasal structures filter out the pollen. 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.