If a single IgM molecule can bind many more antigens than a molecule of IgG, which of the following is higher?

• A. Affinity
• B. Initial force of attraction
• C. Avidity
• D. Initial sensitization

Answer: (C)

Avidity is the overall strength of binding when multiple antigen-binding sites engage with an antigen. IgM is a pentamer, giving it many binding sites, so it can latch onto several epitopes at once. This multivalency makes the total binding much stronger than a single interaction, even if each individual site isn’t as tight as IgG’s. Affinity, by contrast, is the strength of a single binding interaction at one site, which is usually higher for IgG on a per-site basis but doesn’t capture the cumulative grip provided by IgM’s many sites. The other terms aren’t standard measures of binding strength. So the higher measure in this scenario is avidity.