Heterogeneity in breast milk–derived antibacterial IgA reactivity from donors who delivered preterm infants. Donor milk samples (preterm infants; 24–35 wk gestational age) were analyzed with our flow cytometric array (Fig. 1 A). (A) Bar graph showing the concentration of IgA purified from donor milk samples from mothers of term and preterm infants (ELISA). (B) Heat map of normalized antibacterial binding affinity of different preterm donors (Spearman). Samples where no data were collected due to insufficient bacteria in the well are colored gray. Donor numbers indicated on top of each column. (C) Scatter graph showing the normalized antibacterial IgA binding values for each preterm donor (each color represents a different donor). (D) PCA comparing aggregate antibacterial IgA binding between preterm and term samples. This figure is an aggregate of experiments on 15 donor samples.