Infection with type-B-hepatitis virus causes Australia antigen (HB Ag) to appear in the serum. This antigen possesses the "group" specificity, a, common to all HB Ag-positive sera; and one or other of two mutually exclusive subspecificities, d or y, which distinguish the two antigenic subtypes, D and Y. We hypothesize that these determinants, d and y, reflect the activity of two distinct genotypes of hepatitis-B virus (HBV), provisionally designated HBV-D and HBV-Y. Transmission experiments have shown that these viral subtypes "breed true." Their separate identity is further attested by the usual finding of only one subtype in localized outbreaks of hepatitis B. The clinical expressions of HBV-D and HBV-Y infection are beginning to be studied. Early results suggest significant differences in subtype distribution, such as the marked preponderance of Y in drug-abusers' hepatitis.
Le Bouvier GL, McCollum RW, Hierholzer WJ, Irwin GR, Krugman S, Giles JP. Subtypes of Australia Antigen and Hepatitis-B Virus. JAMA. 1972;222(8):928–930. doi:10.1001/jama.1972.03210080020005
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