Affinity vs. avidity

Affinity

Antibody affinity refers to the total strength of non-covalent interactions between the paratope of the antibody and the epitope of the antigen (Figure 1).

Figure1: Antibody affinity

Avidity

Antibodies are bivalent to multivalent, meaning they bind to two or more epitopes at the same time. Antibody avidity (or also called functional affinity) refers to the overall strength of the interaction of multivalent antibodies to their epitopes. In other words, it is the sum of all affinities in the antibody-antigen complex (Figure 2).

Figure 2: Antibody avidity

High avidity can compensate for low affinity. As shown in Figure 3, pentameric IgM has a lower affinity than IgG, but has a higher avidity than IgG due to its multivalence.

Figure 3: Affinity vs. avidity