Question

Explain how high avidity is possible with IgM antibodies despite relatively low affinity for antigen.

Answer 1

Avidity means the strength of interaction between the antibody and a single multivalent antigen.

• This helps to describes the strength with which an antibody binds to a complex antigen.

Affinity means the strength of antigen-antibody interactions at a single antibody binding site.

• Antibodies that bind poorly and have high dissociation rates have low affinity.

High avidity is possible with IgM antibodies despite relatively low affinity for antigen because pentameric structure with 10 binding sites.

• It is the first antibody produced by immune system, as it was the first one to be made it has low affinity and thus there is no affinity maturation. They have high dissociation rates and bind poorly to antibodies.
• High avidity is due to multiple binding sites. It has 10 binding sites forming a pentamer (5) with 2 binding sites per monomer. Hence, even the binding affinity is low, it's avidity is high.