Schematic representation of the hypothetical role of B:b interactions in the lateral aggregation of protofibrils during fibrin polymerization. Designations are the same as in Figure 1. (A) Portion of a protofibril assembled only through A:a interactions with B-knobs exposed on the outer surface of the structure. (B) Portion of a fibrin fiber built of 2 protofibrils aggregating laterally and connected through B:b interactions. (C) Portion of a protofibril assembled through A:a interactions and stabilized by B:b bonds within the protofibril.