Hepatitis B Surface Antigen (HBsAg) is a protein antigen produced by HBV. This antigen is the earliest indicator of acute hepatitis B and often identifies infected people before symptoms appear. HBsAg will disappear from the blood during the recovery period. In some people (those infected as children or those with a weak immune system, such as those with AIDS), chronic infection with HBV may occur and HBsAg will remain positive.