Any natural or synthetic substance capable of eliciting a cellular-signaling response. All allergens are antigens, but the reverse is not true. Antigens bind different classes of Ig receptors (e.g., IgA, IgG, IgM, IgE). Allergens specifically provoke an IgE antibody response (Type I hypersensitivity) in sensitized individuals, while non-sensitized individuals clear the allergen, typically symptom-free, via an allergen-IgG-mediated response.