What is the term for an antigen that causes an allergic hypersensitivity reaction?

The term "allergen" specifically refers to an antigen that triggers an allergic hypersensitivity reaction. Allergens are typically harmless substances, such as pollen, dust mites, pet dander, or certain foods, that provoke an exaggerated immune response in susceptible individuals. This immune response leads to symptoms associated with allergies, including itching, sneezing, and inflammation.

In contrast, a pathogen refers to microorganisms or agents that can cause disease, such as bacteria and viruses, rather than prompting an allergic response. An antibody is a protein produced by the immune system in response to an antigen, and it plays a key role in the immune defense mechanisms but is not an antigen itself. An immunogen is a broader term that encompasses any substance capable of eliciting an immune response, including both allergens and pathogens, but does not specifically denote an allergen in the context of hypersensitivity reactions. Therefore, the term that accurately describes an antigen that leads to allergic responses is indeed "allergen."