An antigen is a substance that triggers an immune response in the body. It can be a part of a pathogen, such as a virus or bacteria, or it can be a foreign substance like pollen or certain foods. When the immune system detects an antigen, it produces specific proteins called antibodies to help neutralize or eliminate the threat. Antigens are crucial for the functioning of vaccines, which contain weakened or inactive parts of a pathogen. This exposure helps the immune system recognize and respond more effectively to future infections by the same pathogen, providing immunity against diseases like influenza or measles.