Primary antibody deficiencies are the most common immunodeficiencies in humans; however, identification of the underlying genetic and biochemical basis for these diseases is often difficult, given that these deficiencies typically involve complex genetic etiologies. In this issue of the JCI, Bouafia et al. performed whole-exome sequencing on a pair of siblings with primary antibody deficiencies and identified genetic mutations that result in a deficiency of ARHGEF1, a hematopoietic intracellular signaling molecule that transmits signals from GPCRs. ARHGEF1-deficient lymphocytes from the affected siblings exhibited important functional deficits that indicate that loss of ARHGEF1 accounts for the observed primary antibody deficiency, which manifests in an inability to mount antibody responses to vaccines and pathogens. Thus, this report demonstrates an important role for ARHGEF1 in GPCR signal transduction required for appropriate adaptive immune responses in humans.