What term refers to a foreign agent capable of causing an immune response in an individual, with repeated exposures leading to progressively stronger abnormal responses?

• A. Foreign agent or substance capable of causing an immune response in an individual
• B. A chemical that causes immediate toxicity
• C. A substance that degrades quickly in air
• D. A material that prevents immune responses

Answer: (A)

The main idea here is recognizing what triggers the immune system. An antigen (often called an immunogen) is a foreign substance that can provoke an immune response in the body. When the immune system first encounters an antigen, it starts a response, and after this initial encounter, the body creates memory cells. On subsequent exposures, those memory cells respond more quickly and strongly, which can manifest as progressively stronger or more pronounced immune reactions.

That’s why this term fits best: it directly names the foreign agent that initiates the immune response. The other descriptions describe a toxic chemical causing immediate harm, a substance that degrades quickly in air, or something that suppresses the immune system—none of which describe an agent that elicits an immune response.