Fig. 4

Several distinct IgA assemblies co-occur in human serum and milk as determined by mass photometry. Mass distribution histograms of IgA assemblies purified from serum (A) or milk (B). Normal distributions were fitted to the histograms depicting counts per mass bin (grey bars with a bin size of 15 kDa). From the fitted normal distributions an average mass was determined for each peak as shown in the legend. For each of the colored peaks a mass and probable composition could be assigned. A table comparing the measured masses with theoretical masses of the different IgA complexes is provided in Supplementary Tables S2 and S3. On the right adjoining the mass histograms are stacked bar charts depicting the relative contribution of distinct IgA assemblies to the total IgA concentration derived from the mass histograms, considering that dimeric J-chain coupled IgA contains twice as many Fab molecules compared to monomeric IgA, etc. C Depiction and description of all probable co-occurring IgA assemblies in human serum (first two structures) and milk (last three structures). Distinctively in serum, about 83% of IgA is present as a monomer, whereas 17% is present as a dimeric J-chain coupled IgA. In milk a distribution of SIgA assemblies is detected, all containing one J-chain, one SC, and two to four IgA molecules. The grey peaks are likely co-purified albumin and unknown (in A, 65 and 227 kDa, respectively), and possibly free SC partially degraded by the acid treatment in purification (in B)