A hepatitis B surface antigen is a protein on the surface of the hepatitis B virus that causes the immune system to produce antibodies, according to the University of Rochester Medical Center. Finding these antigens in the blood is among the earliest signs of a hepatitis B infection.
Antigens are markers produced by bacteria or viruses indicating the presence of these infectants in the bloodstream, explains WebMD. While it can take between 30 and 150 days to develop symptoms of hepatitis B after being infected, these surface antigens can be detected within 12 weeks of the initial infection, reports the University of Rochester Medical Center. Hepatitis B surface antigens indicate an acute infection and remain present in the blood during chronic infections.
The hepatitis B virus causes liver infection and is communicable through blood, seminal fluid and vaginal secretions, states the University of Rochester Medical Center. The virus usually disappears by itself within six months but remains active in up to 10 percent of adults and between 35 and 50 percent of children.
After testing positive for hepatitis B surface antigens, patients are usually tested again after six months, notes WebMD. Positive results indicate a chronic HBV infection that can still spread to others. Chronic hepatitis B infections can lead to liver cell damage, cirrhosis and liver cancer, reports the University of Rochester Medical Center.