What is an allergen?

An allergen is defined as a substance that triggers an allergic reaction in susceptible individuals. When a person who is sensitive to a specific allergen is exposed to it, their immune system mistakenly identifies it as a harmful invader and responds by producing antibodies, leading to symptoms such as sneezing, itching, swelling, or even more severe reactions in some cases. Common allergens include pollen, pet dander, certain foods, and medications.

The other options listed do not encompass this definition: radiation pertains to energy emitted in various forms, a component of blood typically refers to elements like red blood cells or plasma, and although some medications can trigger allergic reactions, medications themselves are not defined as allergens. Thus, the best answer is the one that accurately describes the nature of allergens and their role in allergic responses.