HBsAg is the surface antigen of hepatitis B. The surface antigen is what people used to call “Australian antigen”, which is short for “Australian antigen” and was first discovered in Australia. The intact hepatitis B virus is spherical in shape and consists of an envelope and a nucleocapsid, which contains HBsAg, glycoproteins and other components. There is a protein on the surface of the virus that can be used as an antigen to stimulate the body’s immune system to produce hepatitis B surface antibodies, so it is a marker of the existence of the hepatitis B virus, the first of the five hepatitis B items (i.e. hepatitis B two-and-a-half), if the test shows a positive “surface antigen”, it means that the body is infected with the hepatitis B virus. HBsAg positive person should further check liver function, hepatitis B five and hepatitis B virus nucleic acid (HBVDNA), if HBVDNA positive, it means hepatitis B virus replication, infectious; if both liver function abnormalities and HBVDNA positive, it indicates hepatitis virus activity and should be treated with antiviral therapy. Therefore, HBsAg is a component of the hepatitis B virus envelope, and a positive HBsAg indicates that the body is infected with the hepatitis B virus, and prompt medical attention should be sought.