Low-Molecular-Weight IgM Antibody to Hepatitis B Core Antigen in Chronic Infections with Hepatitis B Virus

IgM antibody to hepatitis B virus core antigen (IgM anti-HBc) develops during acute hepatitis B but frequently persists in chronic infections. To characterize persistent IgM anti-HBc better, 7-88 and 198 immunoglobulin fractions were prepared by rate-zonal centrifugation of sera from 17 patients with persistent hepatitis B (chronic active hepa-titis) and were tested for IgM anti-HBc by a specific radioimmunoassay. In 16sera peak activity was found in 7-88 fractions, although in 11 sera a minor peak was also present in 198 fractions. The low molecular weight of the predominant IgM anti-HBc was con-firmed by gel filtration. In competition experiments, the binding of 7-88 antibody to an anti-IgM-coated solid phase was blocked more effectively by purified IgM than by purified IgG. These findings indicate that hepatitis B carriers with chronic active hepa-titis have predominantly 7-88 IgM anti-HBc and represent a novel demonstration of naturally occurring 7-88 IgM with defined antiviral specificity. The recent development of highly sensitive yet spe-cific antibody-capture immunoassays for IgM an-tibodies to the core antigen (HBcAG) of hepatiti