HBsAg

HBsAg is a surface envelope protein of the Hepatitis B virus and is the earliest detectable serologic marker, appearing before symptoms and elevated liver enzymes during acute infection, typically within 1-10 weeks of exposure. Its presence indicates that the virus is actively replicating and the patient is infectious. In self-limited acute infection, HBsAg typically clears within 6 months, followed by seroconversion to anti-HBs (surface antibody), which confers immunity. If HBsAg persists beyond 6 months, this defines chronic HBV infection, which carries increased risk of cirrhosis and hepatocellular carcinoma. There is a 'window period' between the disappearance of HBsAg and the appearance of anti-HBs during which IgM anti-HBc (core antibody) is the only detectable marker, indicating recent infection. HBsAg is also the antigenic component used in the recombinant Hepatitis B vaccine, which induces protective anti-HBs antibodies without exposure to other viral components. Clinically, HBsAg is used for diagnosis of infection, screening blood donors, monitoring chronic carriers, and assessing response to antiviral therapy (loss of HBsAg is a marker of functional cure). Differentiating chronic active vs. inactive carrier states involves additional markers like HBeAg (marker of high infectivity/replication) and HBV DNA viral load.