Considering these beneficial effects of BCG on innate host defense, it is reasonable to hypothesize that it would be helpful to generate a 'trained immunity status' in hosts defective in T- and B-cell immunity, as they are more vulnerable to infections. However, because of their immune compromised status, these patients cannot be vaccinated with live attenuated vaccines (e.g. BCG). [9] Vaccination with y-irradiated BCG would be possible, but it is not known whether the γ-irradiated vaccine has the same protective non-specific effects on innate host defense. In recent in-vitro experiments, the investigators have demonstrated that y-irradiated BCG can train monocytes in vitro (not published). Therefore, the aim of this study is to determine whether γ-irradiated BCG is able to train the human innate immune system of healthy volunteers in-vivo, just as shown before with the live attenuated BCG vaccine [7].