Antigens and antibodies bind by weak forces such as hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waal’s forces rather than covalent bonds. The strength of this noncovalent interaction between a single antigen-binding site on an antibody and the single epitope is the affinity of this antibody for that epitope.
Antibody affinity is proportionate to the fit of the antigen with its antibody-combining site. Low-affinity antibodies bind antigen weakly and tend to dissociate readily, whereas high-affinity antibodies bind antigen more tightly and remain bound longer.
One crucial effect of booster doses of vaccines is to improve antibody binding by enhancing the affinity maturation process.
The affinity of antibodies for the antigen increases with each successive exposure, known as affinity maturation. Antibody binding improves because mutations occur in the hypervariable region of the DNA that encodes the antigen-binding site (a process called somatic hypermutation). Some of those mutations result in the formation of plasma cells with better antigen-binding sites. Follicular dendritic cells and follicular helper T cells select only those plasma cells with improved antigen-binding sites for clonal expansion.
Secreted pentameric IgM has the highest avidity of the immunoglobulins as its interaction with antigen can involve all 10 of its antigen-binding sites. Because of this reason, despite having a lower affinity than IgG, IgM can bind antigens effectively.
Antibody avidity incorporates the affinity of multiple antigen-binding sites.
The affinity at one binding site does not always reflect the true strength of the antigen-antibody interaction. The avidity of an antibody is a better measure of its binding capacity within biological systems than the affinity of its binding sites.
When complex antigens containing multiple, repeating epitopes are mixed with antibodies containing multiple binding sites, the interaction of an antibody molecule with an antigen molecule at one site will increase the probability of reaction between those two molecules at a second site. The combined binding strength of such multiple interactions between a multivalent antibody and antigen is called the avidity.
IgM often has a lower affinity than IgG, but the high avidity of IgM, resulting from its higher valence, enables it to bind antigens effectively. High avidity compensates for its low affinity.
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