(A) HBsAg[?]HEPATITIS B VIRUS:oThe envelope contains a protein called the surface antigen (HBsAg), which is important for laboratory diagnosis & immunization.oHBsAg was known as Australia antigen because it was first found in the serum of an Australian aborigine.oGenome contains 4 genes that encode 5 proteins:-S gene encodes surface antigen-C gene encodes core antigen & the antigen-P gene encodes polymerase-X gene encodes A protein.oX protein is an activator of viral RNA transcription.oSerological Diagnosis of Hepatitis B virus Infection:oHBsAg and hepatitis B e antigen (HBeAg) are the first markers identified in the serum of patients acutely infected with HBV.oDetection of Antigen:oHBsAg generally appears before symptom onset and peaks during overt disease.oIn patients who successfully clear the HBV infection and do not progress to the chronic carrier stage, HBsAg typically is undetectable 4 to 6 months after infection.oHBsAg (Australia antigen) is the surface antigen of the hepatitis B virus (HBV). It indicates current hepatitis B infection.NOMENCLATURE OF HEPATITIS B*. HBVHepatitis B virus (complete infectious virion)*. 42 nm, double-shelled particle, originally called the Dane particle, that consists of a 7 nm thick outer shell and a 27 nm inner care.*. Core contains a small, circular, partially double-stranded DNA molecule and an endogenous DNA polymerase.*. It is the prototype agent for the family Hepadnavinade.*. HBsAgHepatitis B surface antigen (also called envelope antigen)*. Complex of antigenic determinants found on the surface of HBV and of 22 nm particles & tubular forms. It was formerly designated Australia (Au) antigen or hepatitis - associated antigen (HAA)*. HBeAgHepatitis B core antigen*. Antigenic specificity associated with the 27 nm core of HBV*. HBeAgHepatitis B e antigen*. Antigenic determinant that is closely associated with the nucleocapsid of HBV. It also circulates as a soluble protein in serum.*. Anti HBs, anti-HBc & anti-HBeAntibody to HBsAg, HBCAg & HBeAg*. Specific antibodies that are produced in response to their respective antigenic determinants.[?]HBV Structure & Antigens:oHBV is DNA virus with a double shelloSize is 42 nm in diameteroMost infectious among all hepatitis viruses.oAssociated Antigens:oHBsAg (Australian antigen):-Surface Antigen, Australian antigen-Useful marker form epidemiology-1st marker to appear in blood after infectionoHBeAg: Hepatitis B core antigen; Not detected in serumoHBeAg:-Integral part of core antigen; Enveloped antigen-Indicate active viral replication-so associated with high infectivity.