Re: What causes HIV / AIDS to mess up the imune system?

Hi Andrew,

Thanks for this very interesting question. You are right that the HIV virus inserts its own DNA into the nucleus of a cell. This in itself is not necessarily harmful-the DNA of humans and other organisms is full of "fossil" DNA gained from viruses and other sources long ago. To understand why HIV damages cells, you need to know a little about the life cycle of the virus.

HIV infects a specific type of white blood cell called a T4 helper cell. These are also called CD4 cells, because they have a specific receptor on the surface called CD4. T4 helper cells are critical to the immune system because they activate other immune system cells, e.g. by turning on antibody production.

Viral infection of cells can be divided into several stages. First is attachment, in which a specific protein on the HIV virus binds to the CD4 receptor. Next is fusion, where the virus membrane fuses with that of the cell, allowing the virus inside. HIV is part of a large family of viruses called retroviruses. This means that instead of DNA, it contains a related molecule called RNA. HIV also contains an enzyme called reverse transcriptase, which copies the RNA molecule into a DNA copy.

Once the DNA copy is formed, it can insert itself into the DNA of the T4 helper cell. Now, the integrated DNA "hijacks" the machinery of the cell to produce more of the proteins that make up the HIV virus. These proteins are processed by an enzyme called protease. New viral RNA is also produced from the integrated DNA, and the RNA and proteins are assembled near the surface of the cell.

The final, and critical step, is called budding. The newly-assembled virus surrounds itself with a membrane which it takes from the membrane of the host cell. A large number of viruses then leaves the cell, and this is what kills the T4 helper cell-the holes in the surface caused by the viruses taking away pieces of the cell membrane.

You might like to look at the Cells Alive website, which has a nice description and pictures of the whole process. For some more technical details and other information, have a look at the AIDS 101 website too. Finally, the HIV InSite website is a good source of more technical information.
I hope this helps with your question,

Neil Saunders