Antigens are substances that can trigger an immune response in the body. They are often found on the surface of pathogens like bacteria and viruses, as well as on allergens and even some cancer cells. When the immune system detects an antigen, it produces specific proteins called antibodies to help neutralize or eliminate the threat. In addition to pathogens, antigens can also be present in vaccines, which contain weakened or inactive parts of a virus or bacteria. This exposure helps the immune system recognize and respond more effectively if it encounters the actual pathogen in the future, providing protection against diseases like influenza or measles.