How does HIV cause AIDS?

How does HIV cause AIDS?

HIV affects various parts of the body's immune system. The most important damage it causes is to certain white blood cells known as CD4 cells or T-helper cells. These cells are found in the lymph nodes as well as circulating round the body.

CD4 cells set other parts of the immune system in motion when organisms which cause disease are present in the body. These organisms include viruses, bacteria, protozoa and fungi. They can cause disease if not controlled by the immune system.

The CD4 cells of an HIV-infected person mount a defence against the invading HIV, and it may be held at bay for many years. But the virus is not completely destroyed, and it continues to attack the CD4 cells. Eventually the number of CD4 cells declines and the virus numbers rise.



When the CD4 cells start to decline, the person with HIV becomes vulnerable to other infections. The organisms which cause disease in people with HIV are common organisms, easily kept under control by the healthy immune system. They are able to cause illness only when the immune system is severely damaged, so the illnesses they cause are known as opportunistic infections.

Similarly, cells of the body go out of control all the time, and the immune system keeps them under control. If the immune system is damaged, these cells can cause opportunistic tumours.

Government centres for epidemiology* in the United States have compiled a list of serious illnesses and tumours which may result from immune system breakdown in a person with HIV. Once a person with HIV has experienced one or more of the conditions on this list, they are said to have AIDS.

HIV can also have direct effects upon the body. For example, the virus can attack cells in the brain and impair its working. One result of this is HIV encephalopathy (brain disease), and it is an AIDS-defining condition.

Recent research has shown that taking a combination of anti-HIV drugs (combination therapy) can slow down the effect of HIV on the immune system. When combination therapy is successful, it can improve and sustain the health of a person with HIV and will mean they are less likely to develop AIDS-defining conditions.