hbcag hbcag hbcag (core antigen) is a hepatitis b viral protein. it is an indicator of active viral replication; this means the person infected with hepatitis b can likely transmit the virus on to another person (i.e. the person is infectious). capsid proteinidentifiersorganismhepatitis b virus genotype b2 (isolate vietnam/9873/1997)symbolcuniprotq9qab9search forstructuresswiss-modeldomainsinterpro hepatitis core antigenidentifierssymbolhepatitis_corepfampf00906interproipr002006cath7ablscop27abl / scope / supfamavailable protein structures:pfam structures / ecod pdbrcsb pdb; pdbe; pdbjpdbsumstructure summary schematic overview of the hepatitis b virus particle. hbcag is a constituent of the nucleocapsid core (green hexagon). the genome organisation of hbv. some genes overlap. (orf core, at bottom left, encodes hbcag. structure and function hbcag is an antigen that can be found on the surface of the nucleocapsid core (the inner most layer of the hepatitis b virus). while both hbcag and hbeag are made from the same open reading frame, hbcag is not secreted. hbcag is considered "particulate" and it does not circulate in the blood but recent study show it can be detected in serum by radioimmunoassay. however, it is readily detected in hepatocytes after biopsy. when both hbcag and hbeag proteins are present, it acts as a marker of viral replication, and antibodies to these antigens indicates declining replication. interactions tapasin can interact with hbcag18-27 and enhance cytotoxic t-lymphocyte response against hbv.