Evolution of Immunotherapy Against Pollen Allergy

Allergic diseases have increased worldwide during the last century and are a major disease burden globally. Several substances can induce allergic sensitization and elicit allergic symptoms in sensitized individuals. Pollen grains are one of the main causes of allergic rhinitis and asthma, and the prevalence of different pollen species depends on the climate, geographical regions, flora, and seasons. In addition to avoiding exposure to pollens, anti-allergic drugs are commonly used to mitigate the symptoms of allergies. However, these drugs need to be administered repeatedly as long as the symptoms prevail, usually life-long. Allergen immunotherapy (AIT) is currently the only disease-modifying approach that can prevent the natural progression of the disease (also known as an allergic march), provide a long-lasting therapeutic effect, and prevent the worsening of the symptoms and the occurrence of new sensitizations in allergic individuals. Since the pioneering clinical studies conducted more than 100 years ago using subcutaneously administered pollen extract to treat hay fever, significant advances have been made in the field of AIT. Starting from this pioneering approach, in this review, we have discussed the evolution of the products used for AIT with particular emphasis on pollen allergoids, the chemically modified pollen extracts characterized by lower allergenicity and comparable immunogenicity, and the different routes of administration used for AIT.