Fresh PBMCs were stained with the following conjugated anti-human monoclonal antibodies (mAb): fluorescein isothiocyanate (FITC)–conjugated anti-CD21, energy-coupled dye–conjugated anti-CD19, allophyocyanin (APC)–conjugated anti-IgM, and phycoerythrin (PE)–Cy7–conjugated anti-CD27 (Beckman Coulter). PBMCs were also stained with PE-conjugated anti–FCRL-1, anti–FCRL-2, anti–FCRL-3, anti–FCRL-4, and anti–FCRL-5 mAb. We prepared these mAb as described previously (31 (link)–34 (link)). All of these FCRL-specific mAb were authorized as reference antibodies for FCRL proteins 1–5 (CD307a–e) in previous international human leukocyte differentiation antigens workshops (33 (link),35 (link)). We also used PE-conjugated anti–FCRL-4 mAb (clone 413D12) from BioLegend. PE-conjugated IgG1 (Beckman Coulter) was used as an isotype control. Flow cytometry was performed on a Navios flow cytometer (Beckman Coulter) and analyzed using CXP analysis (Beckman Coulter) and FlowJo software (Tree Star).