Affinity and Avidity

During an immune response, antibodies target and destroy foreign pathogens by binding to specific proteins on the surface of the pathogen called antigens.

These antibody-antigen interactions are critical for preventing the pathogen from infecting other cells and for tagging the pathogens for destruction.

Here, the term affinity refers to the strength of the interaction between a single antibody and a single epitope, or a part of an antigen. For instance, an antibody with a high affinity for a specific antigen will have a strong and stable interaction, preventing dissociation. In contrast, a low affinity interaction will break apart easily.

Varying degrees of antibody affinity allows the immune system to optimize it's response to a given pathogen such as the common cold virus. For example, the first time the immune system encounters a pathogen, the affinity of the antibody-antigen interaction is probably low. However, repeated exposures to the same antigen will cost the host to produce antibodies with increasing affinities to mount a stronger and faster immune response.

Some antibodies can simultaneously bind to multiple antigenic epitopes because they are multivalent. In this case, the cumulative strength of these interactions is referred to as avidity. Typically, the interplay between multivalent antibodies and antigens are high avidity but low affinity.