What is an antigen?

An antigen is defined as a foreign substance that prompts an immune response. Antigens are typically molecules found on the surface of pathogens, such as bacteria or viruses, and can also be present in non-infectious substances, such as pollen or certain foods. When the immune system detects these foreign molecules, it recognizes them as potentially harmful, which activates the immune response. This response may include the production of antibodies by specialized immune cells, as well as the activation of other immune system components to eliminate the threat.

The other choices reflect related concepts but do not define an antigen accurately. While proteins related to blood types are examples of antigens, they do not encompass the full range of antigens that exist. The notion of antibodies pertains to the immune response rather than the triggering substance, and agents involved in blood coagulation refer to different components of the body's hemostatic system, which is distinct from the role of antigens in immunity. Understanding antigens is crucial for comprehending how the immune system functions to protect the body from infections and other foreign invaders.