Kappa chain (V kappa III) subgroup-related activity in an idiotypic anti-cold agglutinin serum

In a search for H- or L-chain-related cross-idiotypic specificity among human anti-I and anti-i cold agglutinins, two idiotypic antisera raised against the IgMkappa cold agglutinin Da were tested for their binding activity to isolated cold agglutinin H and L chains. Negligible H-chain binding activity was found, but there was high-titre L-chain binding activity in one of the antisera. This was an unsuspected VkappaIII subgroup activity which enabled the classification of VkappaIII proteins into three subgroups. The kappa chains of five out of six anti-I and anti-i cold agglutinins belonged to the antigenically most active VkappaIII subgroup. Absorption of the idiotypic antiserum with a Bence Jones protein of this latter subgroup did not appreciably alter the precipitating cross-idiotypic activity of the antiserum when tested with intect cold agglutinins. However, these studies do not rule out the possible existence of a VkappaIII subgroup-associated conformational antigen in an intact Fab region, which is seen as a 'cross idiotypic' antigen by heterologous (rabbit anti-human) antisera.