The electron micrographs (left side) are of negatively stained human fibrinogen complexed with the M1 protein of S pyogenes before (top) and after factor XIII–induced cross-linking (middle). Cross-linking detected with gold-labeled antibody is shown in the bottom panel. Corresponding interpretive schematics are shown on the right side. Taken from Figure 3A in Loof et al, page 2589.¹