Peripheral-blood IgD⁺ CD38^hi T1 B cells are not circulating pre–germinal-center or plasma-cell precursors. (A) Tonsil mononuclear cells were stained with anti-CD19, anti-IgD, anti-CD38, and a fourth mAb to examine the phenotype of IgD⁺ CD38^hi pre–germinal center B cells. (B) Enriched B cells from peripheral blood of a healthy donor were similarly stained to examine the expression of plasma cell markers on CD19⁺ IgD⁺ CD38^hi T1 B cells (solid gate) and CD19^lo IgD^- CD38^bright circulating plasma cells (dashed gate). (C) The relative frequencies of the CD19⁺ IgD⁺ CD38^hi T1 B-cell population (solid gate) and CD19^lo IgD^- CD38^bright plasma cells (dashed gate) from a healthy donor were examined before immunization and 4, 7, and 14 days after immunization with influenza vaccine.