Final answer:
The avidity of IgM is higher than that of IgG even when both have the same individual binding affinity for an antigen, because IgM's multivalency with 10 antigen-binding sites allows for more simultaneous interactions, increasing the overall strength of adherence to the antigen.
Step-by-step explanation:
The question pertains to the comparison between the avidity of IgM and IgG, assuming they have the same affinity for an antigen. Affinity is defined as the strength of a single antibody-antigen interaction, which in the case of IgG, is typically high since each antigen-binding site on IgG has a strong interaction with its target antigen. On the other hand, avidity refers to the overall strength of the combined binding of an antibody, which includes multiple interactions.
IgM is a pentameric molecule, meaning it consists of five linked Y-shaped units, totaling 10 antigen-binding sites. Although each individual binding site of IgM may have a lower affinity when compared to IgG, the multivalent nature of IgM allows it to bind with high avidity to an antigen due to the increased number of interactions that can occur simultaneously.
Thus, despite having the same affinity for an antigen, the avidity of IgM is significantly higher compared to IgG because of the multivalency and the cumulative strength of multiple bindings occurring at one time.