Which of the following describes the term 'antigen'?

The term 'antigen' refers specifically to a substance that triggers an immune response. Antigens are typically foreign molecules, such as proteins or polysaccharides found on the surface of pathogens like bacteria and viruses, that the immune system recognizes as harmful. When the body detects an antigen, it responds by activating immune cells, such as T cells and B cells, to eliminate the threat. This process may also lead to the production of antibodies, which specifically target the antigen for destruction.

The other options, while related to the immune system, do not capture the full meaning of an antigen. A part of a virus may be an antigen, but not all antigens are parts of viruses. Antigens can also be derived from non-viral pathogens like bacteria or even environmental substances like pollen. Similarly, a type of immune cell is a component of the immune response but does not define what an antigen is. Lastly, a medical treatment for infections describes therapies used to combat pathogens rather than the substances that invoke the immune response. Understanding the concept of an antigen is fundamental to grasping how the immune system identifies and tackles foreign invaders.