There are several types of asthma. When caused by an allergen it is known as extrinsic asthma because the causing agent is external. When the air carries allergens (like pollen), sensitive subjects experience an allergic reaction at the level of the bronchial mucosa. This reaction causes a contraction of the muscles surrounding the bronchia, known as bronchial constriction, and an inflammatory reaction that reduces the bronchial internal diameter and causes breathing difficulties. When the reaction disappears, the patient is able to resume normal breathing without fatigue.

As in the case of allergic rhinitis, asthma may appear in a specific season of the year, generally during the presence of the allergen in the air. This is seasonal asthma, like pollen-caused asthma. On other occasions, the allergen is present in larger or smaller proportions throughout the year, causing chronic asthma (for example, mite asthma). In addition, asthma can be associated to allergens existing in specific places or working areas (occupational asthma).

It is important to determine the type of asthma a patient suffers and, if applicable, the causative allergen in order to establish specific treatment. In pollen asthma, symptoms must be treated only during the pollination season. Specific immune therapies (vaccines) are efficient for the majority of extrinsic (allergic) asthma patients.

The allergens that cause allergic asthma are airborne and inhale.