If HBsAg is positive and HBsAb negative, what does that suggest?

• A. Immunity after vaccination
• B. Infectious hepatitis B infection
• C. Immunity from natural infection
• D. Recovery only

Answer: (B)

The key idea is what the serology tells you about current infection and immunity. HBsAg is the surface antigen of hepatitis B; its presence in the blood means the person has an active HBV infection and can be infectious. HBsAb (the antibody to the surface antigen) indicates immunity—either from vaccination or from recovery after past infection. If HBsAb is negative, there is no protective immunity. So having HBsAg positive and HBsAb negative points to an active hepatitis B infection with infectivity and no immunity. Depending on how long the antigen has been present, this could be an acute infection or chronic infection; additional tests (like IgM anti-HBc for acute infection) help distinguish, but the core interpretation is active infection with contagious potential and no immunity.