Influenza is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and can be fatal. The best way to prevent influenza is to get a flu vaccine each year. Getting a flu vaccine helps your body produce an immune response against the influenza virus. Viral antigens include toxins, chemicals, bacteria, viruses, or other substances that come from outside the body. Body tissues and cells, including cancer cells, also have antigens that cause an immune response. These antigens can also be used as markers in laboratory tests to identify the characteristics of those tissues or cells. In this article, we will provide useful information about the antigenic characteristics of influenza viruses.