Have you ever been tested for a viral disease? These are fairly routine tests – you may have had one done before starting a job or perhaps during pregnancy as part of your pre- natal care. If you are a student, chances are you have been tested for an STD as part of a Student union campaign to get young people screened. Or, perhaps you have been in a more urgent scenario where you have been in contact with someone that has TB or suspect that you have caught something whilst away on holiday.
For whatever purpose, patient viral testing is very common and a large portion of the public have been tested either routinely or as a suspected case.
However, have you ever wondered what happens to your sample once it is taken for testing? What magic detects those microscopic pathogens?
As part of my work experience as a Biomedical student I have been fortunate enough to spend a week in a hospital virology department where I have been able to witness first- hand the remarkable work of the lab staff. These people work tirelessly every day, running tests on hundreds of patient’s samples. These could be anything from mouth swabs to blood, urine and faecal samples. It’s not glamorous work, but they work without complaint.
How does disease screening work?
The majority of testing these days is automated so, once the sample has been prepared, it is placed in a machine which accurately detects the presence of a particular pathogen.
Now, for the biochemical part:- Detection is usually based upon one specific molecule present on the viral surface, this molecule is then given the name “antigen”. The way in with this antigen is detected is quite ingenious and makes use of natural biological proteins called immunoglobulins or “anti- bodies”.
Most people are familiar with these molecules as the body’s “defence” mediators, killing off invading germs and foreign bodies. Co-incidentally, immunoglobulins can be used in an entirely synthetic system in order to detect the presence of a virus in the biological sample.
Here’s a breakdown of how immunoglobulins work in the body:
Antibodies are large, Y- shaped proteins which are made up of a “constant” portion and a “variable” portion. The variable portion is found right at the tips of the “arms” of the protein and it uses this portion to bind to the antigen or invading body.
The specificity of the variable portion is crucial for ensuring the immunoglobulin is directed towards the correct target. This variable portion is so unique in its structure that it will only associate with its target and nothing else; very much like how a key will fit one lock. This ensures there is no risk of the immunoglobulin from destroying anything other than the target which might include body tissues.
In contrast with the biological role of immunoglobulins, the ones used in viral detection assays are synthetic molecules. These are designed to have a variable portion that will only associate with the molecule or “antigen” on the virus surface. In the test kits, the immunoglobulins are immobilised to a plate, then the sample is washed over the surface; if the virus is present it will bind to the plate.
The plate is then washed so only the virus remains captured in the immobilised immunoglobulins.
Now, the virus has been captured, a method is required to indicate that it’s there. As you are probably aware, virus cannot be seen with the naked eye so this presents a bit of a problem. The solution is to “flag” the virus by attaching to it a chemiluminescent probe. The luminescence can therefore be identified as the presence of virus.
This “flagging” is mediated again, through the use of immunoglobulins. Two are used this time, the first binds to the viral antigen, the second binds to the immunoglobulin that has bound to the antigen, generating layers, like a sandwich! The second immunoglobulin is the one attached to the probe from which the luminescence is detected. Within a matter of minutes you can determine the result as either negative or positive.
This procedure is an absolute marvel and just goes to show that some of the greatest “tools” exist in our own bodies.
The Advantages of Disease Screening
These test have become so valuable in ensuring the early detection of life threatening diseases and ensuring the appropriate treatment is provided. Also, many of the anti-natal screenings have made it possible to rescue embryos from complications such as severe deformation that can result from maternal infections during pregnancy.
Take a moment to consider what it would be like not to have this service which most of us take for granted. We are so fortunate to have access to this service, not to mention having it free on the NHS. Much of the world’s population still live without access to these health benefits and as a result many severe diseases are still at large among these communities. So, do not take this service for granted! Also, please spare a thought for the people who work to make this service possible. There is a lot at stake when diagnosing a patient. The efficiency and accuracy required of them is huge, without their hard work the service would not be as great as it is.