Molecular analysis of Ig class switching in IgA⁺ and IgG⁺ memory B-cell subsets. (A) Schematic representation of the constant region of the human IGH locus. (B) Distribution of IgA and IgG receptor subclass use in IGH rearrangements of class-switched memory B-cell subsets. Total number of analyzed sequences is indicated in the center of each plot. Differences in the distribution were statistically analyzed with the χ² test and were found significant for both CD27⁺IgG⁺ vs CD27⁻IgG⁺ (P < .0001) and CD27⁺IgA⁺ vs CD27⁻IgA⁺ (P < .05) B-cell subsets. (C) Frequency of Sμ-Sα and Sμ-Sγ rearrangements bearing remnants of indirect class switching. Number of analyzed sequences is given in brackets. (D) Examples of direct and sequential class switching; a piece of Sγ3 sequence in the Sμ-Sγ2 junction is indicated boxed in red font.