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The binding specificity between antibody and antigen drive our immune systems to successfully fight  infection. When a viruses or bacteria invade a body they are engulfed by macrophages, which break them down and present their epitopes to the B cells lymphocytes. These B cells read the epitope and create antibodies with an antigen binding site, or paratope, that specifically recognizes the invading pathogen, binds to it, and signals to the rest of the immune system that the pathogen/antigen should be destroyed. This antibody affinity to antigen is similar to the specificity of a key in a lock.

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Electrostatic interaction in antibody-antigen complex

Antibody affinity is a complicated mix of weak electrostatic interactions including van der waals forces, hydrophobic interactions, ion-dipole affinity, and hydrogen bonding. Only a few amino acids are involved in the interactions, and the binding site is an area of only a few square nanometers in size. This means that the tertiary structure of the antibody is extremely important. If the antibody is exposed to heat it will denature and the unique chemical structure of the antigen-binding site will be altered, In such a case the antibody affinity would be destroyed. This is why it is so important to properly store antibodies when using them for immunodiagnostics and immunotherapy. Additionally, since the affinity between antibody and antigen is governed by weak electrostatic interactions, the binding site is particularly sensitive to changes in pH and ionic conditions of the solvent.

As an antibody paratope nears an antigen epitope the hydration energies of the proteins are overcome and water molecules are expelled from the sites as the antigen-antibody complex is formed. As with any chemical reaction, the antigen-antibody complex is reversible, and an equilibrium will be established between free antibody, free antigen, and the antibody-antigen complex.

Antibody affinity in immunodiagnostics

The antibody affinity for antigen is exploited by immunodiagnostic and immunotherapeutic techniques. We can diagnose disease by identifying surface markers on cells or by detecting the presence of antigens in serum. Traditional techniques such as the ELISA require an enzyme-substrate reaction to identify the formation of antibody-antigen complexes. Newer techniques use chemiluminescence or fluorescent labels, while cutting edge technologies are using optical principles to develop label-free diagnostic strategies.

The future of these label-free diagnostics will bring about lab-on-a-chip designs that will push to create compact technologies for use in remote and resource-poor areas. Microfluidic and photonic techniques will certainly advance this platform throughout the coming decade.  

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Lluis M. Martínez | SEPMAG Chief Scientific Officer

Founder of SEPMAG, Lluis holds a PhD in Magnetic Materials by the UAB. He has conducted research at German and Spanish academic institutions. Having worked in companies in Ireland, USA and Spain, he has more than 20 years of experience applying magnetic materials and sensors to industrial products and processes. He has filed several international patents on the field and co-authored more than 20 scientific papers, most of them on the subject of magnetic particle movement.

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