Let`s dissect each option.

Option 1: An antigen doesn`t lower body temperature. Instead, many antigens (like bacteria or viruses) can cause a fever.

Option 2: Antigens don`t directly destroy harmful bacteria. They are the substances (such as proteins on the surface of bacteria, viruses, or fungi) that trigger the immune system to act against them.

Option 3: This is the correct answer. Antigens are substances that provoke an immune response, involving recognition by white blood cells and producing antibodies to combat the antigen.

Option 4: Antigens are not used as antidotes to poison. Antidotes are medications or other substances that counteract the effects of poison, whereas antigens generally pertain to the immune system`s response to potentially harmful invaders.

In summary, an antigen is a substance that triggers an immune response, rather than lowering body temperature, destroying bacteria, or counteracting poison.