Avidity

Avidity of antibodies is a characteristic of the overall stability of the complex of antigen and antibody. Avidity is determined by the affinity of the antibody to the antigen, the number of antigen-binding centers in the antibody molecule, and the features of the spatial structure of the antigen, which create steric barriers to creating the complex. ^{[1] The} avidity of an antibody should be distinguished from affinity , since affinity is a thermodynamic parameter that quantifies the strength of a single interaction between an antigen and an antibody, while avidity describes the strength of cooperative affinity interactions.

For example, the avidity of IgM can be high even with low affinity of individual antigen-binding centers, since an IgM molecule has ten such centers, while IgG can have two high affinity centers.

The avidity of IgM and IgG is very important in the diagnosis and allows for a retrospective analysis of viral diseases . So, for example, the high avidity of primary IgM indicates an acute phase of the disease and recent - from one to one and a half months - infection. Trace concentrations of IgM can persist in the body, in some cases, up to two years.