Final answer:
An antigen is a foreign substance that elicits an immune reaction by being recognized as non-self by the body's immune system.
Step-by-step explanation:
A substance that produces an immune reaction because it is perceived as foreign to the body is a(n) antigen. An antigen is a macromolecule, often a protein, that is recognized as foreign or "non-self" by the immune system. Antigens bind to specific receptors on immune system cells and can elicit an adaptive immune response. In some cases, B cells respond to these foreign antigens by producing antibodies, which are molecules that precisely match and bind to specific antigens, targeting them for destruction by other immune cells.