Stability of breast milk–derived antibacterial IgA reactivity over the course of sibling infants. Breast milk samples were collected from consecutive siblings and analyzed with our flow cytometric array (Fig. 1 A). (A) Heat map of normalized antibacterial binding affinity of different donors. Hierarchical clustering (Spearman) of various donors is indicated by colored bars above and below the heatmap that correspond to each donor. Donor numbers indicated on top of each column. (B) PCA of aggregate antibacterial samples where each donor is displayed in a different color (from A). The first sibling is indicated by a circle and the second sibling a triangle. Samples colored as in A. (C) Paired Student’s t tests were calculated comparing the IgA binding of each donor between infant one and infant two for each bacterial taxon. The mean change ((Infant 2 − Infant 1; taxa 1) + (Infant 2 − Infant 1; taxa x))/36 (# of taxa) for each paired test was calculated and graphed. Significant increase in second infant = “up” triangle; significant decrease in second infant = “down” triangle; no statistical significance = circle. Colors are according to A. See Fig. S3 for each paired student’s t test. This figure is an aggregate of experiments on 20 samples from 10 donors.