What makes an allergen?

Allergic diseases are an immune disorder reacting to certain type of allergen(s). Remarkably only a small number of proteins of the plant and animal proteome act as allergens. Therefore, allergens have been clustered according to their common structural, biochemical and functional features. Evidence has accumulated that some allergens possess intrinsic adjuvant properties to stimulate the innate immunity. The adjuvant properties appear to contribute to the allergenicity of the respective proteins, namely the ability to cause allergic sensitization in susceptible subjects or allergic reactions in sensitized individuals. Here, we discuss how allergens interact with the innate immune cells, in particular dendritic cells and epithelial cells, via binding to pattern recognition receptors, exhibiting proteolytic activities and/or inducting type 2 innate lymphoid cells (ILC2), thereby contributing to the sensitization and development of allergic diseases.