An antigen is a substance that invokes an immune response in the body. The word is a contraction of the term "antibody-generator," meaning that its presence causes the immune system to create antibodies to respond to a perceived attack or infection. The Australia antigen is a marker of hepatitis, which is a viral inflammation of the liver. The discovery and analysis of the antigen played a crucial part in efforts to fight the disease.
Hepatitis, which comes in many forms, is an dangerous infection of the liver known to cause epidemics that may infect a large number of people. Though scientists had been tracking the disease since the 19th century, the means of discovering the infection were still quite primitive. It was postulated that many patients contracted hepatitis through blood transfusions from those already infected, but the means of identifying the virus hadn't yet been discovered. In the early 1960s, Dr. Baruch Samuel Blumberg, working in concert with Dr. Harvey Alter, first identified the Australia antigen after noting an unusual reaction when the blood from an Aboriginal Australian was added to serum from hemophiliac and leukemic patients.
Though the discovery of this antigen, also known as Aa or HBsAg, was significant, it did not yet link the antigen to hepatitis. Blumberg later made the link between the presence of the antigen and the disease when he retested a child with Down's syndrome who had originally tested negative for the Aa. Months after the initial negative test, the child suddenly tested positive for the Australia antigen, along with developing the symptoms of hepatitis. The connection was later confirmed by repeat instances of the antigen appearing alongside hepatitis, including in one of Blumberg's lab technicians. Eventually, Blumberg's link between the antigen and hepatitis infection demolished the idea that the presence of the antigen was a genetic inheritance, and proved that it was usually synonymous with hepatitis B infection.
Discovering the link between the antigen and hepatitis allowed for the development of a reliable blood-screening test that could check for hepatitis. This became extremely important to the prevention of hepatitis epidemics, as donated blood could now be easily checked for signs of the virus. In many regions, the testing of all donated blood for the Australia antigen became required by law. In addition, the antigen became an important factor in the creation of a hepatitis B vaccine, which uses a heavily filtered version of the antigen to create antibodies in the immune system that will protect against infection.