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Affinity and avidity are terms used to describe the strength of molecular interactions, most commonly in antibody–antigen binding. Affinity refers to the strength of a single binding interaction between an antibody’s paratope and an antigen’s epitope. It is quantified by the equilibrium dissociation constant (Kd); a lower Kd indicates higher affinity. Avidity describes the overall binding strength of an antibody to a multivalent antigen, reflecting the combined effect of multiple binding interactions, valency, and geometry. Because antibodies such as IgG have two antigen-binding sites, and pentameric IgM has ten, multivalent interactions can yield a much greater functional binding strength than the affinity of any one site would suggest.

Difference: Affinity is an intrinsic property of a single binding site; avidity is an emergent property of multiple sites operating together. A high affinity at one site can be offset by low avidity if other sites do not engage; conversely, moderate-affinity interactions can achieve high avidity through multivalent binding.

Measurement and factors: Affinity is typically measured by Kd, kon, and koff; techniques include surface plasmon resonance and isothermal titration calorimetry. Avidity is influenced by valency, epitope density, and spatial arrangement, as well as temperature, pH, ionic strength, and antigen presentation.

Biological relevance: Affinity maturation during immune responses increases antibody affinity for antigen. Avidity maturation can also occur, enhancing overall binding in vivo. Avidity-based tests are used in serology to distinguish recent from past infections, though interpretation depends on the pathogen and assay.