Specific IgM responses correlate with IgM⁺IgD⁺CD27⁺ B cells. (A) Serum IgM recognizing SP capsules, PC, SP TA, and SA TA have positive correlation with percentage of IgM⁺IgD⁺CD27⁺ B cells. (B) Only serum IgG for SP capsules positively correlates with IgM⁺IgD⁺CD27⁺ B cells. P value represents the Spearman rank correlation test. (C) IgM⁺IgD⁺CD27⁺ B cells as a percentage of total B cells. **P < .01.