The correct answer is C. Triggers the immune system.
An antigen is a substance that can trigger an immune response. This means that it can cause the body to produce antibodies, which are proteins that help to fight off infection. Antigens can be found on the surface of bacteria, viruses, and other foreign substances.
Option A is incorrect because antigens do not lower body temperature. This is the job of the hypothalamus, a part of the brain that regulates body temperature.
Option B is incorrect because antigens do not destroy harmful bacteria. This is the job of the immune system, which uses a variety of mechanisms to fight off infection.
Option D is incorrect because antigens are not used as antidotes to poison. Antidotes are substances that are used to treat poisoning. They work by blocking the effects of the poison or by helping the body to remove the poison from the body.