A protein marker found on the core of the hepatitis B virus (HBV). HBV antigen does not circulate in the blood but is found only in fiver cells infected by HBV. HBcAg stimulates the production of a protective antibody, immunoglobulin M (IgM-anti-HBc), which appears in the blood shortly before the onset of symptoms. Tests for this antibody are used with other blood tests in the diagnosis of acute and chronic hepatitis B infection. During the convalescent stage of hepatitis B infection, IgM anti-HBc is replaced by another antibody, IgG antiHBc, which remains in the blood for years.