Hepatitis B is a viral infection of the liver that is transmitted through contact with blood or bodily fluids from an individual infected with the hepatitis B virus (HBV).
The virus mainly affects liver function. It invades the liver cells (hepatocytes) and uses the cells’ machinery to replicate within it. The hepatitis B virion binds to the hepatocyte via the preS domain of the viral surface antigen. The cell then engulfs the virus in a process called endocytosis.
As the infection occurs, the host immune response is triggered. The body’s immune system attacks the infected hepatocytes, which leads to liver injury at the same time as clearing the virus from the body. The liver damage associated with HBV infection is mainly caused by the adaptive immune response, particularly the virus-specific cytotoxic T lymphocytes (CTLs). These CTLs kill cells that contain the virus. Liver damage is also aggravated by the antigen-nonspecific inflammatory cells and activated platelets at the site of infection.
Spread of the infection
Since hepatitis B can be spread through contact with infected blood and bodily fluids, the infection can be transmitted in the following ways:
- Blood transfusion using contaminated blood. Due to routine screening of blood for hepatitis B and other viral infections such as HIV, the risk of hepatitis B transmission via blood transfusion is relatively low these days.
- Hepatitis B can be spread via semen and vaginal fluids and can therefore be transmitted during unprotected sex, including anal and oral sex. Those with multiple sexual partners and new, unknown sexual partners are at an increased risk.
- Drug users who share infected needles are also at an increased risk.
- Hepatitis B can be passed on from an infected mother to her newborn baby during childbirth. This is referred to as vertical transmission and can be prevented by vaccination.
Reviewed by Sally Robertson, BSc