Final answer:
An allergen is any non-infectious antigen causing an allergy, typically a harmless protein substance, and allergies are classified as immediate hypersensitivity responses.
Step-by-step explanation:
Any non-infectious antigen that causes an allergy is known as a Allergen (b). An allergen is typically a harmless protein that can be found in various substances, such as pollen, dust mites, or certain foods like peanuts. When an individual has an allergy, their immune system mounts an inflammatory response to this harmless antigen, treating it as though it were a harmful pathogen. However, in most people, many antigens such as food proteins or pollen do not provoke an immune response, a state known as tolerance. Allergy to pollen is specifically classified as an immediate hypersensitivity (d).