Which of the following is known as a substance that triggers an allergic reaction?

An allergen is specifically defined as a substance that can cause an allergic reaction. Allergens are typically harmless substances that trigger an immune response in sensitive individuals, leading to symptoms such as sneezing, itching, rashes, and even more severe responses in some cases. Common examples of allergens include pollen, dust mites, pet dander, and certain foods.

In contrast, antibodies are proteins produced by the immune system in response to pathogens or allergens, playing a role in identifying and neutralizing these invaders. Pathogens refer to microorganisms that can cause disease, such as bacteria, viruses, and fungi, and are distinct from allergens in that they actively cause illness. Antigens are substances that the immune system recognizes as foreign, which can include both allergens and pathogens, but the term is broader and does not specifically denote those substances that lead to allergic responses.

Thus, the term "allergen" uniquely characterizes substances capable of inducing allergies, making it the correct answer.