24.5
Affinity and Avidity
During an immune response, antibodies target and destroy foreign pathogens by binding to specific proteins on the surface of the pathogen called antigens.
These antibody-antigen interactions are critical for preventing the pathogen from infecting other cells and for tagging the pathogens for destruction.
Here, the term affinity refers to the strength of the interaction between a single antibody and a single epitope, or a part of an antigen. For instance, an antibody with a high affinity for a specific antigen will have a strong and stable interaction, preventing dissociation. In contrast, a low affinity interaction will break apart easily.
Varying degrees of antibody affinity allows the immune system to optimize it's response to a given pathogen such as the common cold virus. For example, the first time the immune system encounters a pathogen, the affinity of the antibody-antigen interaction is probably low. However, repeated exposures to the same antigen will cost the host to produce antibodies with increasing affinities to mount a stronger and faster immune response.
Some antibodies can simultaneously bind to multiple antigenic epitopes because they are multivalent. In this case, the cumulative strength of these interactions is referred to as avidity. Typically, the interplay between multivalent antibodies and antigens are high avidity but low affinity.
Overview
Antibodies bind to toxins or substances on the surface of cells, bacteria, viruses, or fungi. The substance is called an antigen, and the precise binding site is the epitope. The strength of the antibody-epitope interaction is called affinity. When an antibody binds an antigen by multiple epitopes, the cumulative strength of the interaction is called avidity. The strength of the interaction influences the elicited immune response.
The Adaptive Immune System Increases Efficiency by Improving Antibody Affinity
By definition, everything that an antibody can bind to is called an antigen. An antigen can be from another organism, a foreign particle such as a toxin, drug or a physical intruder (e.g., splinter), or the body’s own tissue. The exact point of contact where the antibody binds is called the epitope of the antigen. The strength with which an antibody binds to an epitope is called its affinity.
When the body encounters an antigen for the first time, only some of the available antibodies in the body bind the antigen by chance. The affinity of the antibody is likely low. However, the adaptive immune system earns its name by reacting adaptively to antigens that the organism encounters during its lifetime. Once an antigen has been recognized for the first time, a complex selection process leads to the production of antibodies with higher affinity against this specific antigen. Hence, the affinity of the antibody for a particular antigen is higher when the same antigen is encountered a second time. As a result, the immune response will be stronger.
Antibodies with Varying Affinity or Avidity Carry Different Functions
Some antibodies, such as IgM, possess multiple binding sites that all recognize the same epitope. The cumulative binding strength of such an antibody is called avidity. As a rule of thumb, antibodies with high avidity have low affinity. As a result, IgM more easily recognizes new antigens and can be produced quickly as it does not undergo an elaborate selection process. In fact, IgM is bound to B cells and primarily triggers B cells to produce other classes of antibodies with higher affinity against that newly identified antigen.
During an immune response, antibodies target and destroy foreign pathogens by binding to specific proteins on the surface of the pathogen called antigens.
These antibody-antigen interactions are critical for preventing the pathogen from infecting other cells and for tagging the pathogens for destruction.
Here, the term affinity refers to the strength of the interaction between a single antibody and a single epitope, or a part of an antigen. For instance, an antibody with a high affinity for a specific antigen will have a strong and stable interaction, preventing dissociation. In contrast, a low affinity interaction will break apart easily.
Varying degrees of antibody affinity allows the immune system to optimize it's response to a given pathogen such as the common cold virus. For example, the first time the immune system encounters a pathogen, the affinity of the antibody-antigen interaction is probably low. However, repeated exposures to the same antigen will cost the host to produce antibodies with increasing affinities to mount a stronger and faster immune response.
Some antibodies can simultaneously bind to multiple antigenic epitopes because they are multivalent. In this case, the cumulative strength of these interactions is referred to as avidity. Typically, the interplay between multivalent antibodies and antigens are high avidity but low affinity.
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24.5 : Affinity and Avidity
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