Final answer:
An antigen is a substance that provokes an immune response by signaling the immune system into action, commonly a foreign protein that B and T cells can recognize and respond to.
Step-by-step explanation:
The correct definition of an antigen is a substance that can provoke an immune response upon detection by the body. Specifically, an antigen is usually a foreign or "non-self" macromolecule, typically a protein, that can react with cells of the immune system, such as B and T cells. While the body is constantly exposed to a variety of antigens from the environment, not all antigens will lead to an immune reaction. This is due to the process known as tolerance, where the immune system is regulated to prevent responses against harmless antigens.
Antigens are often found on the surface of pathogens, such as bacteria and viruses, and are recognized as foreign, triggering the immune response. B cells are one of the immune system's responses, producing antibodies that specifically bind to antigens, which can mark pathogens for destruction. Therefore, the correct definition of an antigen is 'B. a substance that sends the immune system into action'.