Written by: Maria Ines Berrojo R.M., Georgia-Vasiliki Gkountana, Javier Sebastian, Ivanna Jovanovic

The immune response is the set of phenomena by which the immune system recognizes foreign elements (antigens) to the body to destroy them through the action leukocytes ( and soluble molecules (e.g. antibodies) that are transported through the bloodstream to all corners of the body. Immunity can be innate (nonspecific) or acquired (specific):

Innate immunity is related to nonspecific mechanisms and is always natural.

Acquired immunity involves the appearance of specific protection mechanisms developed after contact with pathogenic organisms.

Antigens are the molecules that generate an immune response by stimulating the production of antibodies by the B lymphocytes and reacting with them or with the T lymphocytes. They are fundamentally proteins, complex polysaccharides, etc. Within these molecules are antigenic determinants or epitopes, small fragments (five or ten amino acids or hexose units) where it binds to the corresponding antibody

Antibodies or immunoglobulins are molecules made by B lymphocytes in response to contact with an antigen. Chemically they are plasma globular proteins that detect and bind specifically to the antigen, giving rise to the humoral immune response. This allows them to directly detect and neutralize microorganisms, such as bacteria or viruses, or to agglutinate them so that they can be phagocytosed by macrophages.

Each antibody molecule is made up of four polypeptide chains (tetrapeptide) linked by interchain disulfide bridges, in the shape of a Y. They are distinguished:

– Heavy chains (H chains in the figure): each antibody molecule has two identical heavy or larger chains.

– Light chains (L chains in the figure): each antibody has two identical light or minor chains.

Within the chains, different regions are distinguished:

– Variable regions (V regions in the figure): it is located at the NH2-terminal end of each chain, which gives them specificity, since they are different in the different antibodies.

– Constant regions (C regions in the figure): it presents a small number of structures that is characteristic of each class of antibodies.

– Antigen-binding regions: each antibody molecule has two antigen-binding sites, located in the slits between the two chains of each of the Y arms, so that two identical antigens can be linked.

Antibody

Once we dive deep into the state-of-art for a biosensor we must consider what kind of analyte we are aiming to detect Is it a nucleic acid sequence? Is it a protein?

The dangerous Influenza A (H1N1) presents several possibilities: we can detect the specific superficial H1 and N1 antigens. H1 our fella hemagglutinin is a protein-based antigen in charge of binding the virus to the cell, while our fella N1 is an enzyme named neuraminidase in charge of breaking the virus’ connection to surface receptor and getting out of the cell.

Focusing into the external particularities of this virus it is more convenient to employ the following three recognition elements:

Antibodies Nucleic Acids Aptamers

Indeed, these biorecognition elements will bind the analyte with high specificity and sensitivity of the nanobiosensor to make it possible to detect really low concentrations of the virus in Saliva.

References:

Brock Biology of Microorganisms 14th ED. Michael Madigan, John Martinko, Kelly Bender, Daniel Buckley, David Stahl. ISBN: 978-0-321-89739-8 published by Pearson Education 2015

Nucleic acid-based fluorescent probes
and their analytical potential Anal Bioanal Chem (2011) Juskowiak, Bernard 399:3157–3176 DOI 10.1007/s00216-010-4304-5

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