Avidity is the combined strength of multivalent attachment. An intact IgG can make two contacts when accessible epitopes have suitable spacing. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 24, B Cells and Antibodies, A Typical Antibody Has Two Identical Antigen-Binding Sites; The Strength of an Antibody-Antigen Interaction Depends on Both the Number and the Affinity of the Antigen-Binding Sites: avidity
If one contact breaks while the other remains, the detached arm stays nearby and can bind again before the whole antibody escapes. That geometric advantage does not require changing either paratope's intrinsic antibody affinity. Two arms binding two separate freely moving antigens do not automatically provide the same tethering advantage. Epitope spacing, accessibility and surface density matter; avidity is not obtained by adding two dissociation constants. Articles by others on the same topic
Avidity is a term used in various fields, but most commonly, it refers to the strength of the binding interaction between antibodies and antigens. In immunology, avidity describes the overall strength of the binding of an antibody to its respective antigen, taking into account both the affinity of the antibody for a single epitope and the number of epitopes that the antibody can bind.