Hepatitis B is an infectious illness caused by hepatitis B virus (HBV) which infects the liver of hominoidea, including humans, and causes an inflammation called hepatitis. The hepatitis B surface antigen (HBsAg) is most frequently used to screen for the presence of this infection. It is the first detectable viral antigen to appear during infection. However, early in an infection, this antigen may not be present and it may be undetectable later in the infection as it is being cleared by the host. Description: Human plasma. Alternative Names: Hepatitis B Surface Ag Subtype ad protein, Hepatitis B surface Ag protein, Hepatitis B Surface Antigen protein