Australia (Au) antigen,1 also known as serum hepatitis (SH) antigen and hepatitis-associated antigen (HAA), appears in the serum during infection with the agent, or one of the agents, of long-incubation (type B) hepatitis.2,3 Australia antigen is carried on characteristic spherical and tubular particles 20 nm in diameter,4 which are most probably fragments of a viral lipoprotein envelope, related in structure and antigenic specificity to the outer coat of the relatively infrequent 42-nm viruslike particles seen in certain Au-antigen-positive (Au+) sera.5 Whether or not these larger particles represent the actual "hepatovirions" is a matter of continuing debate.
Repeated indications over the past two years suggest that Au antigen is not a single simple entity, and that anti-Au antisera may contain antibody molecules of different specificities. Levene and Blumberg were the first to study these reactions6: they described a common antigenic specificity, "a," shared Au+ sera; and two additional, apparently allternative, specificities,
Le Bouvier GL. Subspecificities of the Australia Antigen Complex. Am J Dis Child. 1972;123(4):420-424. doi:10.1001/archpedi.1972.02110100152059