Gating on germinal center B cells

During adaptive immune responses, lymphocyte activation takes place in the germinal centers (GCs) of secondary lymphoid organs. Dissecting the populations that participate in the GC process is, by definition, difficult. In this issue of Blood, Jackson and colleagues report that a small percentage (10%) of human tonsil B cells expresses the CD45RO (RO+) surface phenotype and contains the highest mutation rate in the V-region genes. These cells are activated centrocytes that still contain transcripts for activation-induced cytidine deaminase (AID), albeit at reduced levels compared with RO- GC B cells.