When HIV gets into your blood stream, the virus attaches to and then enters specific white blood cells in your body known as CD4 cells (also called ‘T-helper cells’). CD4 cells are the white blood cells responsible for organising your immune system’s response to infection. They are therefore an essential part of your immune system, and without them it cannot work properly.

Once inside a CD4 cell, the virus makes more copies of itself (it reproduces, or replicates), and in doing so destroys the CD4 cell. These HIV copies then go on to infect other CD4 cells and the process repeats itself over and over again.

The more CD4 cells HIV infects and destroys, the less effective your immune system becomes. Doctors test for the amount of CD4 cells in your blood to gauge how strong your immune system is. This therefore gives a picture of how much damage HIV has done to your immune system.

The amount of CD4 cells in your blood is measured as the number of cells per microlitre of blood (a microlitre is equal to one millionth of a litre, or one cubic millimetre). This is called your CD4 cell count, or CD4 count. HIV negative people usually have CD4 counts of between 500 and 1,500 cells per microlitre of blood.

The number of HIV copies in your blood is known as your viral load, and is measured as the number of individual HIV copies per millilitre of blood (a millilitre is equal to one thousandth of a litre, or one cubic centimetre). A viral load of over 100,000 copies is considered high, whereas a viral load of under 10,000 copies is considered low.

Your body makes new CD4 cells to replace the ones destroyed by HIV, but there comes a time when the body cannot keep up with the rate that they are being destroyed. This is usually around 8 to 10 years after infection and at this point your CD4 count starts to fall. Having a low CD4 count means that your immune system can’t work properly, leaving you open to opportunistic infections or tumours.

What are opportunistic infections?
Opportunistic infections are caused by organisms (bacteria, viruses, parasites or fungi) that are around us all the time. These organisms are usually kept at bay by our immune systems and so they rarely make us ill. With a low CD4 count, however, your immune system may no longer be able to suppress these organisms, and they are therefore given the ‘opportunity’ to make you ill.

Opportunistic tumours are caused by cells in our bodies multiplying out of control. Our immune systems are usually very good at recognising this and in most cases are able to stop it from happening. If you have a low CD4 count, however, then your immune system will not be as effective in stopping this cell growth, and as such the cells are given the ‘opportunity’ to develop into tumours or cancers.

The risk of developing any of these opportunistic infections or tumors rises significantly if your CD4 count falls below 2001. Therefore, your doctor will want to talk to you about starting treatment for HIV before your CD4 count falls below 350. HIV treatment is very effective in stopping HIV from replicating. In doing so, HIV becomes less able to destroy more CD4 cells, allowing your CD4 count to recover.

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References
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1 CD4 Cell Counts. AIDSMAP, 2010.