HBsAg is the surface antigenof the Hepatitis-B-Virus (HBV). The capsid of a virus has different surface proteins from the rest of the virus. The antigen is a protein that binds specifically on one of these surface proteins. It is commonly referred to as the Australian Antigen. - See more at: http://www.avivasysbio.com/sd/tds/html_datasheet.php?sku=OPPA00881