Nerdy Science Blog

The terms “affinity” and “avidity” confuse many people. They are popular term use in immunology. Both of them mean the binding strength, but what is the difference between them?

“Affinity” is the strength of binding of one molecule to a ligand. For example, the strength of binding of an antibody to an antigen. Affinity is represented by dissociation constant (K_d). K_d is the value the concentration of ligands need to occupy half of the molecules in a solution. In the context of antibody and antigen, it is the concentration of antigens needed to occupy half of the antibody molecules. Smaller K_d indicates that affinity is stronger, hence less antigen is needed to occupy the antibody sites.

However “avidity” is different from “affinity” as avidity is the sum of total strength of binding of more than one molecules to ligands. The difference between them is that affinity is the binding strength of one molecule site to its ligand, but avidity and affinity are correlated. The strength of avidity depends on the affinity and the valency of interactions. For example, immunoglobulin M (IgM) (one type of antibodies, see figure on the right) has a pentameric shape and 10 antigen-binding sites. The multivalent IgM is able to bind to more antigens than other antibodies, such as IgG.

To simply things, affinity is just like how close you can be with a single person. Avidity represents how many friends you can have. For example, you might be attracted to a person in a party and you stick to her for the whole night, and that is the affinity. In the party, you might be able to talk with several people at the same time, and that is called avidity.

References:

Abbas, A.K., Lichtman, A.H. and Pillai, S. (2007), Cellular and Molecular Immunology (6th edn.), Saunders Elsevier.

Murphy, K., Travers, P. and Walport, M. (2008), Janeway’s Immunobiology (7th edn.), Garland Science.

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