Difference Between Affinity and Avidity

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 that are produced by the immune system of the body against the foreign bodies that enter into the body.

Affinity vs Avidity

The main difference between Affinity and Avidity is that Affinity can be defined as the bond strength between the antigen and antibody, whereas comparatively, on the other hand, avidity is the strength of the multiple affinities together. Even the strength of the affinity is low while comparatively, on the other hand, the strength of the avidity is much more powerful or, say, strong.

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 molecule of antibody and antigen relationship.

The three main factors for determining the avidity are – valency, arrangement of the structure, and its binding strength. The antibody IgM is a very good example of Avidity. 

 Comparison Table Between Affinity and Avidity

What is Affinity?

The affinity of a molecule can be stated as the force of attraction between the antibody having its paratope to the antigen having an epitope that is specific to the binding site.

There are four different types of interactions that 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. The examples for 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, the 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

1. 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. 
2. 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.  

Conclusion

The affinity can be defined as the force of attraction between the binding site of the antibody that is paratope and the binding site of the antigen that is epitope.

Thus the binding energy present between the two molecules determines the avidity. Also, avidity is thermodynamically impossibly to explain, unlike affinity, while it can be explained through the kinetic terms of the molecule. 

References

1. https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1097-4644(19960616)61:4%3C554::AID-JCB8%3E3.0.CO;2-N
2. https://www.liebertpub.com/doi/abs/10.1089/cbr.2009.0627
3. https://www.jimmunol.org/content/176/7/4094.short
4. https://www.mdpi.com/2073-4409/10/6/1530