Immunology of viral hepatitis.

So far, five major forms of viral hepatitis, hepatitis A, B, C, D, and E, have been identified. There appears to be at least one other form of enterically transmitted and one other parenterally transmitted hepatotropic virus, but characterization of these viruses is still preliminary. The five hepatotropic viruses have unique structures, yet all the share the property of inducing hepatocellular damage, whether through direct cytotoxicity or through induction of immune mechanisms that lead to hepatocellular necrosis. Advances in molecular biology in the past decade have enabled researchers to understand much about the structure, mechanisms of replication, and viral life cycle of each of these viruses, and successful vaccines have been developed for hepatitis A and B. However, many problems remain unsolved, including which immune system factors are important defenses against these viral infections, which components of the immune system are necessary for a successful vaccine, and what allows some viruses, such as hepatitis B and hepatitis C virus, to become persistent and lead to chronic liver disease.