Surface antigen means infection, surface antibody means protection
Mnemonic
“Surface antigen means infection, surface antibody means protection, core antibody means you met the real thing”:
- HBsAg - the virus itself, so current infection. Over 6 months means chronic
- Anti-HBs - immunity, from vaccination or clearance
- Anti-HBc - only made after natural infection, never after vaccination
That third line is the whole diagnostic trick: vaccinated people have anti-HBs alone; people who cleared the virus have anti-HBs plus anti-HBc.
“e is for extremely infectious”: HBeAg indicates active replication and high infectivity, and its loss usually means lower infectivity, though pre-core mutants are HBeAg negative with a high viral load, so HBV DNA is definitive.
The window period is the trap: after surface antigen clears and before surface antibody appears, IgM core antibody is the only positive marker.
Expansion
| HBsAg | Anti-HBs | Anti-HBc IgM | Anti-HBc IgG | HBeAg | |
|---|---|---|---|---|---|
| Acute infection | + | - | + | - | + |
| Chronic infection | + | - | - | + | + or - |
| Resolved infection | - | + | - | + | - |
| Vaccinated | - | + | - | - | - |
| Window period | - | - | + | - | - |
The three logics
- Surface antigen (HBsAg) is the virus itself, so its presence means current infection. Persistence beyond 6 months defines chronic infection
- Surface antibody (anti-HBs) is the protective response, from vaccination or clearance
- Core antibody (anti-HBc) is only made in response to natural infection, never to the vaccine, so it is the marker that separates the two
e antigen and e antibody indicate replication and infectivity. HBeAg positive means high viral load and high infectivity. Seroconversion to anti-HBe usually means lower infectivity, but pre-core mutants are HBeAg negative with a high viral load, so the HBV DNA is the definitive measure.
The window period is the interval after surface antigen clears and before surface antibody appears, when IgM core antibody is the only positive marker, and it is the classic pitfall.
Isolated anti-HBc may mean a resolved infection with waning antibody, occult infection, a false positive, or the window period, and warrants HBV DNA testing, particularly before immunosuppression, since reactivation with rituximab or chemotherapy can be fatal.