Antibody-antigen interaction
This diagram illustrates the way in which Y-shaped antibody molecules interact with specific antigens. The environment of the antibody’s binding sites, located at the tips of its two arms, provides a good fit just one specific antigen (yellow int his case). Other, differently shaped, antigens do not fit. Each type of virus, bacterium or other pathogen presents a different set of antigens on its surface. The task of the immune system is to ensure that a stock of antibodies with just the right antigen binding site lies ready and waiting for any unwanted antigen that might float past.
To achieve this, a huge range of the B cells mentioned above lie in wait each with thousands of copies of a specific antibody projecting out from its surface. If the specific antigen were to pass by, the antibody binding site would lock onto it, activating a chain of defensive events. The specific B cells needed will multiply rapidly, each one releasing antibodies appropriate for the specific antigen.
In the case of coronavirus, the antibody recognises and latches on to a bit of a protein sticking out on the surface of the virus, known as the spike protein.
As this lovely diagrams shows, by binding in this way to the spike proteins (red), the antibody molecules (green) prevent the virus attaching to the cell.
Antibody binding to the surface of a virus, blocking entry into a human cell
Antibodies can impede pathogens in other ways too, for example by using their two Y-shaped arms to link together with each other or with several pathogens.
This diagrams indicates ways in which antibody molecules and viruses can come together (1) bind to one another in various ways(2) , enabling the stem of each antibody molecule to lock onto a roving phagocyte (yellow)(3) – another type of immune cell, and be ingested by it.
Image credit: Lisa Donohue, CoVPN
Types of COVID -19 vaccine The novel types of vaccine developed during the Covid-19 pandemic use the gene that carries the code for the virus spike protein, rather than a fragment of the protein itself. Different versions of vaccines use different means to get the RNA containing the gene into our cells. The Astra Zeneca, Johnson & Johnson and Sputnik versions cunningly exploit the ability of viruses themselves to penetrate our defences. They make use of a weakened version of a quite different virus: one that causes the common cold in chimpanzees. By exploiting the special ability of viruses to break into cells, this chimp virus penetrates our body’s defences and merges into our cells. Loaded into this helpful virus is the RNA from which our bodies can manufacture the spike protein for the nasty virus. Inside our cells, it deposits its RNA cargo for our cells to make use of, but does no harm itself.
The Moderna and Pfizer-BioNtech version, on the other hand, use an artificial nanoparticle – an oily globule instead of a weakened chimpanzee virus – to get inside our cells.
Permission to show a diagram of such a nanoparticle is being sought. It can be seen on this website with the label LNPs. The vital genetic material from a virus is shown as green spiral molecules. The blue, purple and orange spheres with protruding tails are various oily lipid molecules which protect and support the valuable cargo.
A very different type of coronavirus vaccine does not use RNA at all. The Novavax version uses a more traditional approach in which the spike protein itself is injected in to our bodies. This provokes the immune system directly into developing antibodies to itself.
Whichever way the protein gets into our bloodstreams, the specific antibodies needed to combat the corona virus are created. Crucially, copies of them get stored away – hopefully for a long time – ready to pounce, were the infection to arrive at some future date.
© Andrew Morris 6th March 2021
Getting to Grips with Science 