Pharmacological, single-center, non-profit observational study. The present study is part of a cooperation project between the SC Gastroenterology and Hepatology, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico (Milan, Italy), the University of Milan, the University of Parma and Rome Tor Vergata, funded under the call for Research Projects of Significant National Interest - 2022 PNRR Call (Prot. P2022WEXP2). Hepatitis D virus (HDV) is a defective RNA virus, which requires the presence of hepatitis B virus (HBV) to infect liver cells and propagate. To date, the mechanisms underlying the accelerated disease progression in the natural history of Delta hepatitis are poorly understood, as is the course of the HDV-specific immune response (CD4 and CD8 T cells). As in chronic HBV and HCV infections, the outcome of chronic HDV infection appears to be dictated primarily by the host immune response, which represents a key determinant for virus control or persistence. For HBV/HDV coinfection, the role of T cells has not been well defined, as suitable animal models are lacking and so far few HDV-specific T cell epitopes have been precisely mapped, mainly limited to HLA-B alleles. The study is divided into two substudies (cross-sectional and longitudinal). The primary objective of the cross-sectional study is to calculate the prevalence of HDV-specific T responses in patients with chronic HBV-HDV infection naïve to treatment with Bulevirtide. The primary objective of the longitudinal study is the change in the prevalence of HDV-specific T responses in patients with chronic HBV-HDV infection during treatment with Bulevirtide compared to baseline (pre-treatment).