Asthma is a heterogeneous and very complex group of diseases, which includes different clinical phenotypes depending on symptoms, progression, exacerbation pattern or response to treatment among other characteristics. The allergic phenotype is the most frequent, especially in pediatric asthma. It is characterized by sensitization (production of specific IgE) to allergens, and frequent comorbidity with rhinitis and atopic dermatitis. An allergen is a substance that triggers the production of specific IgE antibodies in susceptible individuals, who present symptoms that will depend on the type and intensity of the allergenic load and the tissue where the interaction occurs. Airborne allergens cause their effects in the respiratory tract and eyes, and can be indoor or outdoor, perennial or seasonal. Although allergens such as mites, pollens or animal dander are generally considered as a single particle, it is important to note that they contain different molecules which could trigger distinct specific IgE molecules on different patients.The key points that a physician who treats asthma should be aware of are: