Flow cytometric gating algorithm demonstrating circulating CD27⁺ B-cell subsets in patients after HSCT. Gated CD19⁺ B cells were first examined for IgD and CD38 expression (far left column). Subsequently, each subset (a-d) was analyzed for CD27⁺ expression according to Sims et al.⁴³  (a*) IgD⁺ Memory B cells are IgD^HiCD38^LoCD27⁺. (b*) Post-GC memory B cells are IgD^LoCD38^LoCD27⁺. (c*) Pre–germinal center (GC) B cells are IgD^HiCD38^HiCD27⁺. (d*) Plasmablast or plasma cell–like (PB/PC) B cells are IgD^LoCD38^HiCD27⁺.