HBsAg is the surface antigen of the Hepatitis-B-Virus (HBV). It inicates current Hepatitis B infection. 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. Today these antigen-proteins can be a genetically manufactured (e.g. transgene E.coli) to produce material for a simple antigen test, which finds out the presence of HBV.