What exactly is HBsAg and HBsAb? What exactly is the difference between HBsAg and HBsAb? Are they antibodies that protect against the HBV or is it the actual virus?

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Joëlle Share
Aug 19, 2016


Ag is the antigen and Ab is the antibody.


First it is important to know the difference between and antibody (Ab) and an antigen (Ag):

  • Antibody = protein produced by the immune system to 'neutralize' all (toxic) molecules foreign to the body.
  • Antigen = a foreign and/or toxic molecule that induces an immune response.

Now the difference in this example:

  • HBsAb = Hepatitis B surface antibody that is produced because the body has been exposed to the Hepatitis B virus (HBV).
  • HBsAg = Hepatitis B surface antigen, this is the part of the virus that induces an immune response.

Presence of either HBsAb and/or HBsAg mean different things:

  • Presence of HBsAb = the body has been exposed to HBV. It usually appears about one month after the virus has disappeared. This means someone is no longer contagious when HBsAB is present. Also, it protects the body from getting HBV in the future.
  • Presence of HBsAg = early sign of an active infection with HBV, people are contagious in this stage.
  • Presence of both HBsAb and HBsAg = sometimes this occurs and means that the body is fighting off the infection, but people are still contagious.

There is also another test for HBV that tests for the presence of the Hepatitis B e-antigen (HBeAg). This antigen is only present during an active HBV infection. It can be used to determine how contagious someone is and to determine the effectiveness of treatment.

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