Following entry into the body, HBV is carried to the liver by the bloodstream. The mechanism by which HBV causes liver injury is not known but most likely results from the body's immune system attacking the infected liver cells. After entering the liver cell, the virus replicates by a mechanism involving reverse tran-scriptase (figure 24.20), unusual for a DNA virus. The double-stranded viral DNA genome is transported to the host cell nucleus, where messenger RNA and a RNA copy of the genome are synthesized. This RNA copy of the genome is subsequently transcribed into DNA by reverse transcriptase, and when a double strand is formed by DNA polymerase, this DNA becomes the genome for the newly forming virions. Double-strand formation ceases when the virions bud from the host cell, however, generally leaving part of the genome single-stranded. ■ reverse transcriptase, p. 356

HBsAg appears in the bloodstream days or weeks after infection, often long before signs of liver damage are evident. In 1% to 6% of cases, the infection smolders in the liver, unsuspected for years. About 40% of chronically infected people eventually die from cirrhosis, meaning scarring, of the liver, or from liver cell cancer. Evidence indicates that these cancers result from HBV transformation of liver cells. ■ transformed cells, p. 358

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