What is the Difference Between Affinity and Avidity

The main difference between affinity and avidity is that affinity is the strength of a single interaction of an antibody, whereas avidity is the strength of accumulation of multiple affinities. Furthermore, affinity occurs when a single antigen binds to a single region of an antibody while avidity occurs with the occurrence of multiple binding interactions. 

Affinity and avidity are two types of binding strengths created in an immune complex. Generally, both types of interactions contribute to the agglutination process. 

Key Areas Covered 

1. What is Affinity
     – Definition, Interaction, Importance
2. What is Avidity
     – Definition, Interaction, Importance
3. What are the Similarities Between Affinity and Avidity
     – Outline of Common Features
4. What is the Difference Between Affinity and Avidity
     – Comparison of Key Differences

Key Terms 

Affinity, Agglutination, Antibody, Antigen, Avidity 

What is Affinity 

Affinity is the strength of the interaction of a single antigenic epitope with a single paratope of an antibody. Generally, it is a type of interaction that occurs during agglutination. Significantly, it requires a high degree of specificity of antibodies to bind to a particular epitope. Usually, ABO antibodies show such specificity while binding to their antigenic counterpart. Furthermore, these interactions are non-covalent bonds.  

What is Avidity 

Avidity is the strength of a collection of affinities. Generally, it occurs through the simultaneous interaction of multiple antigen-binding sites with target antigenic epitopes. Basically, the formation of a single epitope-paratope interaction increases the likelihood of other interactions to occur. On the other hand, in multiple interactions, although a single interaction is broken, the presence of other many interactions does not allow to diffuse the antigen away from the antibody.

Figure 1: Affinity vs Avidity

Furthermore, in general, each antibody contains at least two antigen-binding sites. Therefore, antibodies are usually either divalent or multivalent. As an example, IgM is an antibody with low affinity. However, it has ten weak binding sites, increasing the binding strength. In contrast, IgG, IgE, and IgD are antibodies with two stronger binding sites, and hence, they tend to produce single binding affinities.  

Similarities Between Affinity and Avidity 

• Affinity and avidity are two types of interactions which produce strengths or forces in an immune complex. 
• They are important in the agglutination process. 

Difference Between Affinity and Avidity 

Definition 

Affinity refers to the attraction which a specific antibody possesses to its corresponding antigen while avidity refers to the strength, which an antigen-antibody immune complex possesses at that epitope.  

Significance 

Moreover, affinity is the strength produced by a single epitope interaction with a paratope on the antibody while avidity is the strength produced by several affinities.  

Strength 

The strength of affinity is low, while the strength of avidity is high. 

Specificity 

Antibodies require a high specificity towards its epitopes in affinity while antibodies have a less specificity towards their antigens in avidity.  

Number of Antigen-Binding Sites 

Antibodies with high affinity are monovalent or divalent while antibodies with avidity are multivalent. 

Example Antibodies 

ABO antibodies, IgG, IgE, and IgD antibodies form affinity, while IgM forms avidity interactions.  

Conclusion 

Basically, affinity is the strength produced by an interaction of a single epitope to a single paratope on the antibody. On the other hand, avidity is the strength produced by multiple affinities. Therefore, the strength of affinity is higher than the strength of avidity. However, the main difference between affinity and avidity is the type of interaction between antigen and antibodies.   

References

1. Rudnick, Stephen I, and Gregory P Adams. “Affinity and avidity in antibody-based tumor targeting.” Cancer biotherapy & radiopharmaceuticals vol. 24,2 (2009): 155-61. doi:10.1089/cbr.2009.0627.

Image Courtesy:

1. “Figure 42 03 04” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia   

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