(a) The molecule acting as the receptor is coloured in light blue (heavy chain) and pink (light chain), respectively, with its long axis oriented horizontally. The heavy and light polypeptide chains in the antigen molecule are coloured cyan and violet, respectively. (b) The self-recognition by subset no. 2 BcR IGs is dominated by light-chain-mediated contacts. Two ordered solvent molecules, shown as red spheres, provide bridging interactions between epitope and paratope. Carbon atoms in the ‘antigen’ light chain are coloured violet, pink in the receptor light chain and light blue in the receptor heavy chain. (c) Analysis in solution of subset no. 2 BcR self-association. Dimeric species of the P11475 Fab (with expected sedimentation coefficient 5S) cannot be detected at protein concentrations of 20 μM in SV AUC experiments, consistent with a low-affinity interaction and fast dissociation rate. (d) Dilution ITC allow the detection of the weak homotypic complex. Top panel shows the raw calorimetric dilution titration profile for the subset no. 2 P11475 Fab fragment, the bottom panel the fit of the integrated heat of dissociation with KD=430±160 μM and ΔH0=−5.4±0.6 kcal mol−1.