The hepatitis B virus (HBV) belongs to the Hepadnavirus family. Its members are strongly species-specific and tissue-specific, a property called tropism, so each virus infects only a narrow range of hosts and tissues. The only human-related member is HBV, which mainly infects the human liver and, to a lesser extent, the pancreas and kidney.
The virion is very small, about 42 nm across, and is called the Dane particle. It is stable to freezing, mild heat, ether and low pH, and this stability is one reason person-to-person transmission is so efficient.
The Dane particle has an outer envelope and inner contents. The envelope forms the surface of the particle and contains glycoproteins called surface antigens, or HBsAg. These occur in three forms — large (L), medium (M) and small (S) — all encoded by the same gene but translated from different AUG start codons, and all sharing the same C-terminal end. Inside the envelope lies the icosahedral capsid, built from the core antigen HBcAg. Inside the capsid sit a protein kinase and the polymerase, which carries reverse transcriptase activity and ribonuclease H activity.
HBV particles reach very high concentrations in the blood: including the non-infective excess 22 nm particles and the complete 42 nm virion, they can reach 10 trillion in each millilitre. Some of these particles are secreted into the serum of the infected patient during replication rather than staying in the cell:
- HBeAg, which is very similar to HBcAg in protein structure but is mainly secreted into the serum, where it carries several different antigenic parts.
- HBsAg-containing particles, which are immunogenic and became the first commercial vaccine against HBV.
To hold the particle together, the peri-capsid is made up of three proteins — large, medium and small HBsAg — while the capsid itself is built from HBcAg, the repeating building block of the icosahedral structure.