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To understand how HIV damages your immune system, it helps to know what the virus looks like.

• At the center of the virus is a strand of RNA (short for ribonucleic acid).
• The RNA is the blueprint (or genetic code) that the virus uses to make more copies of itself. It contains just nine genes. (You have about 30,000 genes.)
• Small spikes of proteins stick out of the surface of the virus. Doctors call them gp120 antigens.
• These protein spikes fit exactly onto the surface of your CD4 cells.

Here's what happens when HIV gets into your blood and meets a CD4 cell.

• The protein spikes on the surface of HIV stick onto certain receptors on the surface of the CD4 cell.
• These receptors are called CD4 receptors.

• When HIV has stuck to the CD4 receptor, it can get inside the CD4 cell.

• Once inside the cell, HIV does something that only a few viruses can do. It changes its RNA into DNA, which stands for deoxyribonucleic acid.
• It does this using a special chemical called reverse transcriptase.
• The virus's DNA can get inside the center part (the nucleus) of your CD4 cell.
• Inside the nucleus, the virus DNA becomes part of the CD4 cell's own DNA.
• The HIV DNA can stay in the human DNA for many years.

• At some point, the virus DNA "wakes up" and starts to make extra copies of the HIV RNA.
• The new pieces of RNA act as the blueprint for many new copies of HIV.
• When they are finished, the new viruses pop out from the CD4 cell. Along the way, the CD4 cell dies, but we're not sure exactly how.
• Millions of new viruses are made in this way. The new viruses move on to infect other CD4 cells.
• So many of your CD4 cells are killed when HIV makes new copies of itself. Without CD4 cells, your body can't fight off infections.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.