Affinity and Avidity are the terms that are connected with the study of the immune system that is immunology. The term is linked with the antibodies produced by the body’s immune system against the foreign bodies that enter the body.
- Affinity measures the strength of a single binding site between a ligand and a receptor; avidity refers to the overall binding strength between a multivalent ligand and its target.
- Affinity measures the binding strength of one binding site; avidity considers the cumulative strength of multiple binding sites.
- Avidity is more relevant for evaluating the functional strength of interactions involving multivalent ligands and receptors.
Affinity vs Avidity
Affinity is measured by the equilibrium dissociation constant (Kd), which is the concentration of antigen required to dissociate 50% of antibody-antigen complexes. Avidity is the strength of the interaction between a multivalent antibody and a multivalent antigen, determined by the affinity of the individual binding sites.
Affinity, in simple words, can be stated as the bond strength between the antigen and the antibody. The only point that must be considered is that affinity is determined for a single antibody and antigen relationship molecule.
The three main factors for determining avidity are – valency, the arrangement of the structure, and binding strength. The antibody IgM is a very good example of Avidity.
|Parameters of Comparison||Affinity||Avidity|
|Definition||The attraction between the antibody and its specific antigen||Attraction or the strength between the multiple affinities|
|Significance||Between the epitope and the paratope||Produced by multiple affinities|
|Antigen-Binding Number||Monovalent or Divalent||Multivalent|
|Example||IgD, IgE, IgG, and ABO antibodies||IgM|
|Expressed In||Thermodynamic terms||Kinetic terms|
|Terms of calculation||Single||Multiple|
What is Affinity?
The affinity of a molecule can be stated as the force of attraction between the antibody having its paratope and the antigen having an epitope specific to the binding site.
Four different types of interactions help in participating at the binding site – hydrogen bonds, hydrophobic bonds, van der Waal forces, and electrostatic bonds.
Even the term is measured or expressed in the form of thermodynamic terms. Examples of affinity are the antibodies IgG, IgE, IgD, and ABO antibodies.
What is Avidity?
Antibodies tend to have multiple binding sites on their surface that may be up to 2 to 10 in number. Thus avidity is the sum of the strength of the multiple affinities given for the multiple antibodies with multiple binding sites.
There are three different factors that affect or influence avidity, and they are – the structure of the arrangement, the binding strength, and the valency.
In simple words, avidity can be explained as the individual sum of the affinity of the multiple paratopes with its specific binding antigen or epitope. The capacity of the IgM antibody is five times more than that of IgE, and it is the best example of avidity.
Main Differences Between Affinity and Avidity
- The affinity of a molecule can be expressed through thermodynamical terms, whereas comparatively, on the other hand, the avidity of a molecule can be expressed only through kinetic terms.
- The terms of calculation of affinity are based on a single molecule of antibody to its specific binding antigen, whereas comparatively, on the other hand, the terms of calculation of avidity are based on antibody with multiple sites to antigen with multiple sites.
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Piyush Yadav has spent the past 25 years working as a physicist in the local community. He is a physicist passionate about making science more accessible to our readers. He holds a BSc in Natural Sciences and Post Graduate Diploma in Environmental Science. You can read more about him on his bio page.