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. These are the counter-action produced by these antibodies, and their ability to join with antigens is known as affinity, while the strength is known as avidity.
Affinity vs Avidity
The 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. Initially, the strength or the affinity of the paratope is low, but when they get familiar with the immune response, they automatically gain it.
An avidity, to be precise, is stated as the capacity of the antibody to bind with the different antigen molecules, and that is what avidity is. 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
| 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 |
| Strength | Low | High |
| Specificity | High | Less |
| 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 to the antigen having an epitope that is specific to the binding site. But the main point of the affinity is that it is the strength calculated for the single-molecule of the antibody that is present.
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. All the four interactions are mainly present in between the two different molecules, and thus their presence also influences the affinity.
Initially, the affinity of the molecule is low, but with the given time, the immune system of the body gets used to it, and the affinity increases. 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. The other name used for the term is functional affinity.
There are three different factors that affect or influence avidity, and they are – the structure of the arrangement, the binding strength, and the valency. For example, the antibody IgM that is present in the body has ten paratope or binding sites on its surface.
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
- Affinity is defined as the capacity of the single antibody molecule that can bind with the other antigen molecule in simple words, the relationship between the antibody and antigen, whereas comparatively, on the other hand, the Avidity of the molecule is defined as the capacity of many antibody molecules or can say, the sum of affinity to be precise.
- The significance of the affinity is that it is the strength of the single paratope present at the antibody molecule that can bind with the epitope of its specific antigen, whereas comparatively, on the other hand, the significance of the avidity is that it is the sum of the affinity of the multiple antibody molecules.
- The affinity of the molecule is low, whereas comparatively, on the other hand, the avidity of the molecule is high.
- The specificity of the single antibody molecule towards its antigen to be needed is very high, whereas comparatively, on the other hand, the specificity of the avidity towards its multiple antigens is low.
- Different antibody molecules have different affinities, thus making the antibody molecule with greater affinity to be monovalent or divalent towards the binding of the antigen, whereas comparatively, on the other hand, the antibodies possessing avidity are mainly divalent towards antigen binding.
- Some of the examples of the affinity are – IgG, IgE, IgD, and the ABO antibodies, whereas comparatively, on the other hand, the example for the avidity is the IgM antibody present in the body.
- 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.
Conclusion
To summarize the above topic, the two terms affinity and avidity both seem to look similar to each other, but instead of that, they are way too different from each other. 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.
Whereas on another side, avidity is the force of attraction between the multivalent (having multiple binding sites called paratope) antibody with the multivalent (multiple binding sites called epitope) antigen. 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.
