Antibodies are a major component of the immune response and a powerful tool in research and disease therapeutics. But characterizing antibody-secreting cells in terms of their antibody production rate and the specificity and affinity of the antibodies they make has not been possible at the single cell level. In a recent paper in Nature Biotechnology, Jean Baudry and colleagues report a method that enables massively parallel characterization of single IgG-secreting cells. The microfluidic approach encapsulates individual cells in droplets and immobilizes them onto an array. Magnetic nanoparticles are used to establish an in-droplet immunoassay with a fluorescent readout that allows quantifying the different parameters. The authors show examples of how the method can be used to characterize the dynamics of humoral immune responses in vaccinated animals. These findings and other potential applications of the approach could inform vaccination strategies and identification of useful antibodies. A News and Views commentary from George Georgiou and colleagues accompanies the paper.