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Affinity vs Avidity

A diagram of an antibody. There is a label for affinity and a label for avidity, pointing to different parts of the molecule.
Whilst affinity represents the binding strength between one paratope and one epitope, avidity represents combined strength of all binding sites on a single antibody molecule. Credit: Technology Networks
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Read time: 4 minutes

Affinity and avidity are two terms used in immunology and microbiology to describe binding strength between an antibody and antigen. Because both affinity and avidity involve binding strength, the two terms are often confused.

In this article, we will highlight what affinity and avidity are, how they function in the human body, the differences between the two and how they are exploited in scientific techniques.

What are antibodies and antigens?

Every day the human body is bombarded by hazardous substances and microorganisms. Antigens encompass pathogens or irritants that threaten the normal functioning of the body.

Common antigens include microorganisms like viruses, fungi, and bacteria and toxins like pesticide residue and heavy metals. These external antigens are known as foreign antigens because they originate outside of the body. There are also autoantigens, or self-antigens, which are substances produced inside of the body that prompt an immune response.

Once detected, antigens invoke an immune response that works to protect the body from antigen-induced damage.

A central part of this immune response is the creation of antibodies. Antibodies are molecules produced by the immune system to fight antigens.

Antibodies are created when B lymphocytes, or B cells, come into contact with an antigen. B cells are specialized white blood cells that respond to toxins, microorganisms, and other threats.

The difference between affinity and avidity.While affinity represents the binding strength between one paratope and one epitope, avidity represents combined strength of all binding sites on a single antibody molecule. Credit: Technology Networks

Antibodies are released into the lymphatic system to scour the body for the identified antigen. Every antibody is a y-shaped protein. The tip of each y-shaped arm contains one or more antigen binding sites, called paratopes, that attach to a specific portion of the antigen’s surface, called the epitope.

When an antibody encounters one of the antigens that triggered the immune response, it must bind to it in order to destroy or neutralize it and eliminate the threat. This is where affinity and avidity come in.  

What is affinity?

Affinity is the strength of a single bond or interaction. When it comes to the antibody-antigen relationship, the binding affinity is the strength of the interaction between the antigen’s epitope and the antibody’s paratope at a singular binding site.

Thermodynamically, affinity is the total of all forces that result in increased binding strength (Kon) minus all of the forces that result in decreased binding strength (Koff).

When an antigen is encountered for the first time, the affinity of the antibodies produced is low. Once the body is familiar with the antigen, the immune response adapts and the binding affinity increases.

Another example of affinity is the interaction between a biomolecule and its ligand, such as a protein and a drug designed to bind to it.

Types of affinity

The non-covalent interactions that participate in the affinity of a binding site include:

  • Hydrogen bonds
  • Electrostatic bonds
  • Van der Waals forces
  • Hydrophobic forces

These interactions at the binding site between the two molecules and the presence of other molecules influences the affinity.

What is electron affinity?

When discussing affinity in relation to the antibody-antigen relationship, we are not referring to electron affinity. Electron affinity is defined as the amount of energy released when an electron is added to a neutral atom (in the gaseous phase).

In comparison, the affinity between antigens and antibodies is caused by numerous chemical interactions between the two molecules.

What is avidity?

Antibodies and antigens are multivalent, meaning they possess more than one binding site. The measure of the total binding strength of an antibody at every binding site is termed avidity. Avidity is also known as the functional affinity. 

Avidity is determined by three factors.

  1. The binding affinity: The strength of the relationship at a singular binding site.
  2. The valency: The total number of binding sites involved.
  3. The structural arrangement: The structure of the antigen and antibody involved.

Let’s take an antigen-antibody complex for example. Two of the antibodies found within the human body are IgE and IgM. IgE antibodies have just two binding sites, while IgM antibodies have ten.

The affinity of each of these antibodies is the strength at just one binding site, while the avidity is the total strength of the binding interactions at the two IgE binding sites or the ten IgM binding sites. The valency of IgM is five times greater than that of IgE, so the difference between the affinity and the avidity will be greater for IgM antibodies than IgE antibodies.

Another factor that can change the avidity is the structural arrangement of either the antibody or antigen. For instance, if a non-competitive inhibitor binds to the IgM antibody, its avidity towards its antigen would decrease.

Affinity vs avidity

Affinity and avidity are both measures of binding strength. While affinity is the measure of the binding strength at a single binding site, avidity is a measure of the total binding strength.

Antibodies have between two and ten binding sites. Antibodies with fewer binding sites tend to have high affinity and low avidity, while those with greater binding sites tend to have low affinity and high avidity.  

How are affinity and avidity exploited in scientific techniques?

Affinity and avidity tests are valuable tools in research and medicine. Some of the common applications include:

  1. As a diagnostic tool: Avidity assays are used in diagnostic medicine. Avidity tests help to exclude or identify a variety of pathogens. Avidity tests are currently used to detect and learn more about HIV (human immunodeficiency virus), rubella virus, hepatitis viruses and more.
  2. In pharmaceutical research: Binding affinity is a central component of developing new pharmaceuticals. For instance, binding affinity tells researchers how a drug candidate interacts with the desired target, an indicator of how effective it would be at treating certain diseases.    
  3. In cancer research and treatment: Avidity and affinity tests are used to identify the activity and success of anticancer therapeutics, particularly with regards to tumor targeting.
  4. Chromatography: Affinity chromatography is a method used for purification of a specific molecule or group of molecules from a mixture. By using the relationship between two molecules, such as the affinity between an antigen and antibody, scientists can purify desired biological molecules.