The nucleocapsid of hepatitis B virus is covered by an envelope of HBV surface antigen (HBsAg), which has the common, group-specific determinant a. The four major subtypes of HBsAg, adw, adr, ayw and ayr, are generated, and they have been proposed to represent the phenotypic expression of the four major genotypes of HBV1. HBsAg subtypes show distinct geographical distributions2 and may provide historical information on the migration of ancestors carrying HBV of a particular subtype3.
The HBsAg, localized on the surface of the infectious Dane particle is considered to be the antigen responsible for conferring immunity to infection with hepatitis B4. A larger amount is found on filaments of 20-nm diameter and variable length. By far the most HBsAg is present on small, noninfectious, 20-nm particles5.
Refences:
1.Le Bouvier, G. L., Mccollum, R. W., Hierholzer, W. J., Irwin, G. R., Krugman, S. & Giles, J. P. (1972). Subtypes of Australia antigen and hepatitis-B virus. Journal of the American Medical Association 222, 928-930
2.Courouce-Pauty, A. M., Plancon, A. & Soulier, J. P. (1983). Distribution of HBsAg subtypes in the world. Vox sanguinis 44, 197-211.
3.Yamashita, Y., Kurashina, S., Miyakawa, Y. & Mayumi, M. (1975). South-to-north gradient in distribution of the r determinant of hepatitis B surface antigen in Japan. Journal of Infectious Diseases 131, 567-569.
4.A. M. Moriarty, B. H. Hoyer et al. Expression of the hepatitis B virus surface antigen gene in cell culture by using a simian virus 40 vector. Proc. Nati. Acad. Sci. 78: 2606-2610(1981).