Science Questions

How do you test for AIDS?

Sun, 2nd Nov 2008

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Question

Mattea, San Francisco asked:

I recently listened to one of your podcasts on AIDS and the possible vaccine based on plant proteins. One of the questions that intrigued me was if AIDS hides in our genome how do AIDS tests work?

Answer

Chris - You’re right. HIV, when you become infected with it, it infects a class of cells that have markers on their surface called CD4 cells. That can include white blood cells, macrophages and a whole other class of immune cells. Some of those cells become what’s called productively infected. So the virus goes in, hijacks a cell and turns it into a virus factory. Not all cells have that happening in them. In some cells the virus goes in, it doesn’t turn it into an immediate virus factory. What it does is it makes a DNA copy of the virus RNA and integrates that DNA copy or inserts the DNA copy of the virus inside your own DNA. Then it just turns off and so you have cells wondering around your body that contain HIV and they can turn on that HIV when they want to or when the signals are right for that to happen. To all intents and purposes they’re just a cell going about their daily business. How do you know that person’s got HIV? There are tests that we do in the laboratory to detect HIV are what’s called serological tests. One test will look for antibodies because although people don’t seem to become immune to HIV they nonetheless make huge numbers of antibodies against different parts of the virus. We run a blood test in which you take a sample of the patient’s blood and you present that blood sample with various proteins which are made synthetically but they’re based on what’s on the surface of a virus. They look for whether antibodies in the patient’s blood can bind or lock on to that surface with the viral coat on it. If that happens it means that it’s a reaction test and it goes positive and we can tell. Another way to do it is if people are just acutely infected, they’ve only just been infected with HIV they may not have made any antibodies by that stage. That sort of test would miss that. There’s another kind of test which looks instead for virus antigen. When the virus is growing in cells it’s producing lots of virus proteins which get spat out by cells and they go round the bloodstream. You can do various tests which do the reverse of the test I just described. They have antibodies on the surface of the test plate. Those antibodies grab out of the blood any virus proteins. You can then detect that they’ve been picked up and that gives you a positive. There’s two ways to do it. It’s all done indirectly by markers. There’s a third way which is actually doing it by DNA tests. You can take a sample of a patient’s blood and you can then do PCR – polymerase chain reaction – and you can try to amplify or copy virus DNA and it only copies if the virus is present. You can detect how much virus is there and you can detect virus that’s lurking inside your own genome.

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