Although the majority of memory B cells and class switched memory B cells are derived from the donor (71.7% and 99.1%, respectively), the absolute number of donor-derived memory B cells and class switched memory B cells were 7.9/ L (1.1% of total B cells) and 3.3/ L (0.45% of total B cells), respectively, which was substantially lower than that in age-matched control and consistent with insufficient immunoglobulin production (Fig. Although the absolute count was still lower than that in normal individuals, donor-derived memory B cells and class switched memory B cells has increased to 56/ L (5.2% of total B cells) and 33/ L (3.1% of total B cells), respectively, at 5 years post HCT. Our patient achieved B-cell reconstitution 4 years after HCT, with the co-presence of successful T-cell reconstitution with increased and differentiated donor-derived B-cells and nonfunctional recipient-derived B-cells. [Extracted from the article]