Hepatitis B virus (HBV) is a human pathogen, causing serious liver disease. At the center of the hepatitis B virus is DNA, which contains the genes the virus uses to replicate itself. Surrounding the DNA is a protein called HBcAg (hepatitis B core antigen), which cannot be detected with blood tests. Surrounding this is HBsAg, which is actually part of the protective "envelope." This envelope surrounds the virus and protects it from attack by the body's immune system. HBsAg stands for hepatitis B surface antigen and is the surface antigen of the Hepatitis-B-Virus (HBV) S-gene. 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.