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the “bonus binding strength” that comes when a binding partner binds with multiple binding sites (is multivalent). Basically, avidity makes it so that the strength you measure (the effective or apparent affinity) is greater than if you were to add up the individual affinities of each site. It can arise because binding to one site kinda tethers the others nearby making it easier for them to bind (effectively increasing the concentration of the binder near the molecule to be bound). It comes into play with things like antibodies (which have multiple binding sites for antigens) and lectins (which have multiple binding sites for carbs).