Isolation of IgA from breast milk by Peptide M columns substantially enriches for secretory IgA. (A) The soluble fraction of a breast milk sample was run over a Peptide M column and analyzed on an LDS-PAGE gel. Shown are the Peptide M bound fraction, the flow through, and the aqueous phase prior to separation. MW, molecular weight. (B) LDS-PAGE gels were prepared as in A, transferred to nitrocellulose, and blotted with antibodies to different proteins as described above the blot. L = ladder; M+ = Peptide M bound fraction and M− = flow through of Peptide M column. (C) Concordance of IgA estimations made my 280 nm light absorbance and IgA ELISA. (D) 10-fold dilutions of Peptide M purified IgA fractions run over the flow cytometric array as described in Fig. 1 A. Shown are two example bacteria. A and B are one example of two separate experiments. C is the aggregate of all of the donor samples used in the manuscript. D was performed once. Source data are available for this figure: SourceData FS1.