What is the definition of an antigen?

An antigen is defined as a substance that is recognized by the immune system as foreign, prompting an immune response. These substances can be proteins, polysaccharides, or even smaller molecules, and they are present on the surface of pathogens like bacteria and viruses, as well as on the surface of other foreign cells or particles. When the immune system detects an antigen, it triggers the production of antibodies and activates various immune cells to combat the perceived threat.

Choosing the option about a toxic particle that stimulates an immune response accurately captures the role of antigens in the immune system's function. While harmful bacteria or viruses (other options) can indeed carry antigens, they are more specific types of antigens rather than providing a comprehensive definition. Similarly, while vaccines often contain antigens to prepare the immune system for future infections, calling a vaccine component an antigen does not encompass the broader definition of antigens themselves. Thus, the defining characteristic of antigens as substances that provoke an immune response is best encapsulated in the selected definition.