What characterizes an allergen?

An allergen is characterized as a substance that triggers an immune response in sensitive individuals. When someone who is allergic to a specific substance is exposed to it, their immune system mistakenly identifies this normally harmless substance as a threat, leading to reactions that can range from mild to severe. This immune response can manifest in various ways, such as hives, difficulty breathing, or even anaphylaxis, which is a life-threatening condition.

The other options do not accurately define an allergen. For example, while many allergens may be common foods or additives, not all commonly consumed foods are allergens. Furthermore, a harmless additive does not provoke an immune response, distinguishing it from an allergen. Lastly, while some allergens may be chemicals found in foods, not all chemicals in foods are classified as allergens. The pivotal factor that defines an allergen is its ability to induce an immune response in predisposed individuals.