Pro-GC cells that are located in the germinal center. (A) Anti-IgD (red), anti-AID (green), and anti-CD38 (dark blue) were used to stain frozen tonsillar sections. Lymphoid follicles were identified as naive (red), germinal center (green), pre–germinal center (cyan appearance from combination of red, green, and dark blue), and IgD⁺CD38⁻AID⁺ (yellow appearance from combination of red and green). IgD⁺CD38⁻AID⁺ cells are labeled by a white arrow. Six representative sections are shown. (B) Pro-GC cells most likely exist as a transitory population between naive and germinal center cells because of their cell surface and molecular characteristics described here. We propose that pro-GC cells lie at a critical junction in which AID expression and somatic hypermutation (SHM) begin to take place but the cell has not yet matured with a full complement of germinal center surface markers. Because of cell size and the dynamics of CD69 and CD23 expression in the tonsil subpopulations, we place Bm2 prior to Bm1 in this diagram.