Final answer:
An antigen is a macromolecule found on pathogens that causes immune responses; it serves as a unique identifier which the immune system can recognize and respond to. Vaccines contain antigens to induce immunity without causing disease.
Step-by-step explanation:
An antigen is a portion of a pathogen that has a specific shape and can cause an immune response. Antigens are macromolecules that have the capability to interact with the immune system. A pathogen's unique molecular structures, which are recognized by immune cells, are called antigens.
For example, when someone receives a vaccine, the substance within the vaccine contains a modified version of the virus or bacteria (the pathogen) that does not cause disease, but nonetheless provokes an adaptive immune response. This response is due to the presence of the antigen from the pathogen. As a result of this immune response, the person develops immunity to the future infections by that pathogen thanks to the memory lymphocytes formed during the immune response.