How AIDS Works

Once HIV enters the body, it heads for the lymphoid tissues, where it finds T-helper cells [source:The Body]. Let's look at how the HIV virus infects immune systemcellsand replicates.

Binding:First, HIV attaches to the immune cell when the virus's gp120 protein binds with the CD4 protein of the T-helper cell. The viral core enters the T-helper cell, and the virion's protein membrane fuses with the cell membrane.

Reverse transcription:The viral enzyme, reverse transcriptase, copies the virus's RNA into DNA.

Integration:The newly created DNA is carried into the cell's nucleus by the enzyme, viral integrase, and it binds with cell's DNA. HIV DNA is called a provirus.

Transcription:The viral DNA in the nucleus separates and creates messenger RNA (mRNA), using the cell's own enzymes. The mRNA contains the instructions for making new viral proteins.

Translation:The mRNA is carried back out of the nucleus by the cell's enzymes. The virus then uses the cell's natural protein-making mechanisms to make long chains of viral proteins and enzymes.

Assembly:RNA and viral enzymes gather at the edge of the cell. An enzyme called protease cuts the polypeptides into viral proteins.

Budding:New HIV particles pinch out from the cell membrane and break away with a piece of the cell membrane surrounding them. This is how enveloped viruses leave the cell. In this way, the host cell is not destroyed.

The newly replicated virions will infect other T-helper cells and, left untreated, cause the person's T-helper cell count to slowly dwindle. The lack of T-helper cells compromises the immune system. If a person's T-helper cell count drops below 200 cells per cubic millimeter of blood, he or she is considered to have AIDS. Untreated, a person with AIDS has a life expectancy of three years [source:疾病预防控制中心].

HIV infection follows three basic stages. Usually developing within two to four weeks, the first stage is known asacute HIV infection. The infected person can experience symptoms similar to the flu, including rashes, fevers and headaches. At this point, the virus is multiplying quickly throughout the body. It's during this stage that the infected person is the most contagious.

The second stage is calledchronic HIV infectionorasymptomatic HIV infectionorclinical latency. The virus's rate of multiplying reduces to a low level, and symptoms often disappear. Untreated chronic HIV infection usually develops into AIDS within 10 to 12 years.

The third and final stage of HIV infection is AIDS itself, but no one dies from AIDS specifically. Instead, an AIDS-infected person dies from infections because the immune system has been dismantled. If left untreated, people with AIDS could die from thecommon coldas easily as fromcancer.