Hepatitis B surface antigen

Hepatitis B surface antigen (HBsAg) is a protein on the surface of the Hepatitis B virus (HBV); it is one of the earliest markers of acute hepatitis B and frequently identifies infected individuals before symptoms appear. HBsAg is also referred to as Australia antigen, named after its discovery in the blood serum of an Australian Aboriginal person in the 1960s. The presence of HBsAg in the blood typically indicates that the person is infectious and can spread the virus to others through their blood and body fluids.

Overview

Hepatitis B is a potentially life-threatening liver infection caused by HBV. It can cause chronic infection and puts people at high risk of death from cirrhosis of the liver and liver cancer. HBsAg is one of several antigens produced by HBV; its detection in the blood is the earliest sign of acute hepatitis B infection. Testing for HBsAg is a routine part of prenatal screening and the diagnosis of hepatitis B infection.

Structure and Function

HBsAg is a complex protein that exists in several subtypes, which vary geographically. The antigen is part of the viral envelope and plays a crucial role in the entry of the virus into the liver cell. It is also involved in the assembly and secretion of the virus from infected cells. The immune response to HBsAg is a critical determinant of the outcome of hepatitis B infection; the presence of antibodies against HBsAg (Anti-HBs) usually indicates recovery and immunity from HBV infection.

Clinical Significance

The detection of HBsAg in the serum is a primary method to diagnose acute and chronic hepatitis B infections. A positive HBsAg test result indicates that the person is potentially infectious. The test can also be used to monitor the effectiveness of treatment for hepatitis B infection. In individuals vaccinated against hepatitis B, the presence of anti-HBs antibodies, rather than HBsAg, indicates immunity to the virus.

Diagnosis

Testing for HBsAg is done using blood samples. The most common methods include enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA), and chemiluminescent immunoassay (CIA). These tests are highly sensitive and can detect HBsAg within weeks of infection, often before the patient shows any symptoms.

Treatment and Prevention

There is no specific treatment for acute hepatitis B; care is supportive. For chronic hepatitis B, antiviral medications can help to fight the virus and slow its ability to damage the liver. Prevention of hepatitis B infection revolves around vaccination, which is effective in preventing the infection and its chronic consequences. The hepatitis B vaccine induces the production of anti-HBs, providing immunity against the virus.

Epidemiology

Hepatitis B is a global health problem, with over 250 million people worldwide living with chronic hepatitis B infection. The prevalence of HBsAg varies significantly from one region to another, with the highest rates in sub-Saharan Africa and East Asia. Vaccination has significantly reduced the rate of new infections, especially in countries with universal vaccination policies.

See Also

• Hepatitis B virus
• Hepatitis B vaccine
• Cirrhosis
• Liver cancer