Gut bacteria segregate into IgA^bright and IgA^low fractions in healthy humans. Related to Figs. 1 and 2. (A) The secretory component binds a modest range of carbohydrates. Glycan reactivity with the secretory component was assessed using glycan microarray technology (660 structures). Representative median RFUs are shown. Glycans specifically recognized by secretory component are colored in red. (B) Anti-IgA1 and anti-IgA2 antibodies do not cross-react. Flow cytometry analysis of IgA1 and IgA2 expression on peripheral B cells from a healthy donor. (C) IgA-coated bacteria split into IgA^bright and IgA^low fractions depending on IgA1 and IgA2 coating. Representative flow cytometry analysis of IgA1 and IgA2 coating in IgA^bright-coated bacteria (red lines), IgA^low-coated bacteria (blue lines), and IgA-unbound bacteria (gray lines). (D) Representative flow cytometric analysis of colon and ileum microbiota (left and central panels, respectively) with anti-IgA FITC or isotype-matched control antibody, as indicated. Numbers indicate percentage of positive cells. Data are cumulative from three independent experiments. Boxes extend from the 25th to the 75th percentiles. Error bars represent minimum and maximum values. P values were defined using the Mann-Whitney test. ***, P < 0.001. SSC-A, side scatter area. Quantification of IgA-coated bacteria in stool (n = 20) and ileum (n = 5).