P-Rex1 is a guanine-nucleotide exchange factor (GEF) that activates the small G protein Rac, thus regulating a wide range of physiological and pathophysiological responses. Neutrophils are leukocytes of the innate immune system in which P-Rex1 regulates several Rac-dependent responses, especially those elicited by GPCR signalling. The aim of my project was to assess the functional importance of a new interactor of P-Rex1, the GPCR adaptor protein Norbin, in neutrophils. Norbin is an essential neuronal protein that binds directly to GPCRs, regulating GPCR signalling and trafficking, through unknown mechanisms. Our laboratory recently identified that Norbin stimulates P-Rex1 Rac-GEF activity and promotes P-Rex1 membrane localisation. Furthermore, we showed that Norbin is expressed in myeloid cells. To investigate, we generated mouse strains with conditional deletion of Norbin in myeloid cells and with combined Norbin/Prex1 deficiency. Unexpectedly, I found increased adhesion, spreading, ROS production and degranulation responses in isolated Norbin-deficient neutrophils. Moreover, Norbin-deficient neutrophils had an increased ROS-dependent capacity to kill Staphylococcus aureus. In vivo, Norbin deficiency provided increased immunity against pulmonary infection with Streptococcus pneumoniae, even in immune-deficient Prex‒/‒ mice. Neutrophil depletion showed that Norbin deficiency renders neutrophils important for combatting this infection. Mostly, Norbin deficiency overrode the functional impairments caused by Prex1 deficiency, although some neutrophil responses remained P-Rex1-dependent. Mechanistically, the Norbin deficiency caused constitutive upregulation of some GPCRs onto the neutrophil surface and promoted the GPCR-dependent activities of Rac and Erk, whereas several other signalling pathways and the surface levels of adhesion molecules were not obviously affected. Together, my data indicate that the GPCR adaptor and P-Rex1 regulator Norbin plays an important role in suppressing the host defence functions of neutrophils. A subset of Norbin functions are P-Rex1 dependent, whereas others are likely mediated through other regulators of Rac and of Erk, as well as through the control of GPCR trafficking.