Updated project metadata. The phagocyte NADPH oxidase is critical for host defence, and EROS (Essential for Reactive Oxygen Species) protein is indispensable for expression of its gp91phox-p22phox heterodimer. EROS deficiency in humans is a recently described cause of chronic granulomatous disease (CGD), but its mechanism of action remains unknown. We show EROS binds the immature 58kDa gp91phox directly prior to glycosylation and heme incorporation, thus preventing its degradation. EROS interacts with the oligosaccharyl transferase complex (OST) present at the endoplasmic reticulum. Inhibition of the OST complex suppresses EROS-mediated stabilisation of the 58kDa, providing insight into the early steps of gp91phox biosynthesis. EROS also regulates the purine receptors P2X7 and P2X1 through direct interactions. Accordingly, lack of EROS results in markedly abnormal P2X7 signalling, inflammasome activation and T cell responses. The loss of both the phagocyte respiratory burst and P2X7 signalling lead to resistance to influenza infection. Our work identifies EROS as the first described highly selective chaperone controlling key proteins in innate and adaptive immunity. It has profound implications for immune physiology, immunodeficiency and gene therapy.