Allergic symptoms are triggered by exposure to allergens in sensitised individuals. Allergy is specific in the sense that only allergens to which the individual is sensitised provoke symptoms. The most important allergens are the most common proteins present on airborne particles in the air we breathe. The particles are inhaled and upon contact with the moist surface of the airway mucosa, proteins are extracted and come into contact with the immune system. Allergen exposure can also occur through the gastrointestinal tract or through the skin. Bee and wasp allergens are present in the venoms of the insects and come into contact with the immune system after injection into the body.
The most important allergens on a worldwide scale are house dust mites and grass pollens. Equally important on a regional basis are birch pollen in Northern and Central Europe, parietaria and olive pollens in Southern Europe, ragweed in the United States, and cedar pollen in Japan. Cat and dog allergens are of intermediate importance on a global scale. Locally, other inhalation allergens may dominate.