Final answer:
Avidity is greater than affinity when multiple binding interactions occur between an antibody and an antigen, as seen with multivalent antibodies like IgM. Despite low affinity at individual binding sites, the overall avidity is high due to the multiple interactions.
Step-by-step explanation:
Avidity is higher than affinity when multiple interactions are involved in antibody-antigen binding. For example, IgM molecules, which are secreted as pentameric structures, can have low affinity at each antigen binding site but exhibit high avidity due to the multivalent nature of their structure allowing multiple bindings at once. This concept illustrates the difference between affinity, the strength of a single antibody-antigen interaction, and avidity, the combined strength of all interactions within an antibody-antigen complex.
Although individual IgG molecules are known to have high affinity binding sites, it is the collective strength of all interactions - the avidity - that becomes greater. When an antibody-like IgM, which has ten antigen binding sites, comes into play, the multitude of binding opportunities compensates for the lower affinity, thereby making the overall avidity high.