Functional affinity constants of subfragments of immunoglobulin G for Clq

The functional affinity constants for Clq of subfragments if IgG1 representing the C gamma 2 (c gamma 2III) region or the whole C gamma 3 region of Fc (pFc'), have been measured by examining the ability of these fragments to inhibit the interaction between radiolabelled Clq and glutaraldehyde-treated human erythrocytes or aggregated human IgG. The value of the functional affinity constant for the C gamma 2III fragment was the same as that for Fc and that determined previously for monomeric IgG, indicating that all the elements necessary for Clq binding are contained in a single C gamma 2 domain. The pFc' fragment was inactive but a more degraded trypsin fragment from this region, at C gamma 3, showed the same affinity of binding for Clq as the C gamma 2III and Fc. These results confirm earlier findings that it is not combination of residues in the C gamma 2 which bind Clq which is responsible for their activity but their accessibility.