What term describes a foreign substance that triggers an immune response in the body?

The term that describes a foreign substance that triggers an immune response in the body is an antigen. Antigens are typically proteins or polysaccharides found on the surface of pathogens, such as bacteria and viruses, as well as on non-infectious substances like pollen or certain food proteins. When these antigens are recognized by the immune system, they stimulate the production of antibodies and activate various immune cells to mount a defense. This is crucial for protecting the body from infections and diseases, as the immune system learns to recognize these foreign substances and remember them for future encounters.

In contrast, the other terms listed have different meanings. A pathogen refers to a microorganism or virus that can cause disease, while an allergen is a specific type of antigen that causes allergic reactions, leading to hypersensitivity in some individuals. Antibodies, on the other hand, are the proteins produced by the immune system in response to antigens and they help neutralize or eliminate the threat posed by these foreign substances. However, antibodies themselves are not foreign substances; they are part of the body’s immune response. Thus, the correct answer is clearly centered on the role of antigens in initiating an immune response.