Hepatitis B virus (HBV) is a viral cause of hepatitis. This Liver disease can be prevented by active or passive immunization, and is highly infective through parenteral and sexual routes. It is the most thoroughly characterized and complex etiologic agent. The infective Dane particle consists of a viral core plus an outer surface coat. The outer surface can be detected in serum by immunologic means as hepatitis B surface antigen (HBsAg), formerly Australia antigen.
At least three distinct antigen-antibody systems are intimately related to HBV:
1. HBsAg is associated with the viral surface coat; its presence in serum is usually the first evidence of acute HBV infection and implies infectivity of the blood. HBsAg characteristically appears during the incubation period, usually 1 to 6 wk before clinical or biochemical illness develops, and disappears during convalescence. The corresponding protective antibody (anti-HBs) appears weeks or months later, after clinical recovery, and usually persists for life; thus, its detection indicates past HBV infection and relative immunity. In up to 10% of patients, HBsAg persists after acute infection, and anti-HBs does not develop; these patients usually develop chronic hepatitis or become asymptomatic carriers of the virus.
2. Core antigen (HBcAg) is associated with the viral core. It can be found in infected liver cells but is not detectable in serum except by special techniques that disrupt the Dane particle. The antibody to HBcAg (anti-HBc) generally appears at the onset of clinical illness; thereafter, titers gradually diminish, usually for years or life. Its presence with anti-HBs is not significant beyond indicating previous HBV infection. It is also regularly found in chronic HBsAg carriers, who do not mount an anti-HBs response. In chronic infection, anti-HBc is mainly of the IgG class, whereas in acute infection, IgM anti-HBc predominates. Occasionally, IgM anti-HBc is the only marker of recent HBV infection, reflecting a "window" between disappearance of HBsAg and appearance of anti-HBs.
3. The e antigen (HBeAg) appears to be a peptide derived from the viral core. Found only in HBsAg-positive serum, HbeAg tends to parallel the production of viral DNA polymerase. Its presence, therefore, reflects more active viral replication and is generally associated with greater infectivity of the blood and a greater likelihood of progression to chronic liver disease. In contrast, presence of the corresponding antibody (anti-HBe) points to relatively lower infectivity and usually portends a benign outcome.
The serology associated with hepatitis B can be confusing. An example of the serology and its interpretation are as follows:
|Diagnosis||Surface antigen (HBsAg)||Suface antibody (HBsAb)||Core antibody (HBcAb; IgM, IgG)||E antigen (HBeAg)||E antibody (HBeAb)||HBV DNA|
|Acute hepatitis B||+||-||+ (IgM)||+||-|
|Chronic carrier state||+||-||+||-||+||-|