The HBV core 18–27 peptide (HBcAg18–27), which is one of the most well-characterized markers of protection against HBV, was originally described as a human leukocyte antigen (HLA)-A2.1-restricted CTL (cytotoxic T lymphocyte) epitope and was detected in more than 90% of HLA-A2.1 patients who resolved acute HBV infection.