Although the existing paradigm states that CMV reactivation is under control of cellular immune response, the role of humoral counterpart is underestimated. Anti-CMV positive woman after conditioning with Bu-Flu-ATG underwent stem cell transplantation from fully matched, seronegative sibling donor. In immunodeficient recipient fast and significant decrease of specific immune response was observed but reconstitution of marrow-derived B and NK cells was prior thymic origin T cells. The lowest CMV-IgG(89 U/ml) was observed just before CMV viremia. Noteworthy, the sole and exclusive factor of CMV-specific immune response is a residual recipient antibody class IgG. The CMV-quantiferon increase was observed later, but in the first phase immune reconstitution of the PHA-induced IFNγ release was significantly lower than that CMV-induced. It corresponds with NK cells increase at the top of lymphocyte reconstitution and undetected CMV-specific CD8 cells by pentamer technique. Most of NK cells are CD16+, thus are stimulated by residual IgG. In immunocompetent donor the cellular and humoral immune response increases in parallel manner but symptoms of CMV mononucleosis were observed till the increase of specific IgG. Our observations shed light on a unique host-CMV interaction: indicate that significant decrease of CMV-IgG is a good predictor for CMV reactivation during secondary immunodeficiency.