Treatment with convalescent plasma has been shown to be safe in COVID-19 infection, although efficacy reported in immunocompetent patients varies. Nevertheless, neutralizing antibodies are a key requisite in the fight against viral infections. Patients depleted of antibody-producing B cells, such as those treated with rituximab (anti-CD20) for haematological malignancies, lack a fundamental part of their adaptive immunity. Treatment with convalescent plasma appears to be of general benefit in this particularly vulnerable cohort. We analysed clinical course and inflammation markers of three B-cell depleted patients suffering from COVID-19 that were treated with convalescent plasma. In addition, we measured serum antibody levels as well as peripheral blood CD38/HLA-DR positive T-cells ex vivo and CD137 positive T-cells after in vitro stimulation with SARS-CoV-2 derived peptides in these patients. We observed that therapy with convalescent plasma was effective in all three patients and analysis of CD137 positive T-cells after stimulation with SARS-CoV-2 peptides showed an increase in peptide-specific T-cells after application of convalescent plasma. In conclusion, we here demonstrate efficacy of convalescent plasma therapy in three B cell-depleted patients and present data that suggests that while application of convalescent plasma elevates systemic antibody levels only transiently, it may also boost specific T cell responses. This article is protected by copyright. All rights reserved.