HBsAg is a surface antigen for Hepatitis B virus of subtype Ay (HbsAg-Ay) of 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. Description: Human Plasma. Alternative Names: Hepatitis B Surface Antigen protein, Hepatitis B surface Ag protein, Hepatitis B Surface Ag Subtype Ay protein