The Life-cycle of HIV
Like all viruses, HIV treads the fine line that separates living things from nonliving things. Viruses lack the chemical machinery that human cells utilize to support life. So, HIV requires a host cell to stay alive and replicate. To replicate, the virus creates new virus particles inside a host cell and those particles carry the virus to new cells. Fortunately the virus particles are fragile.
Viruses, like HIV, don't have cell walls or a nucleus. Basically, viruses are made up of genetic instructions wrapped inside a protective shell. An HIV virus particle, called a virion, is spherical in shape and has a diameter of about one 10,000th of a millimeter.
HIV infects one particular type of immune system cell. This cell is called the CD4+T cell, also know as a T-helper cell (see How the Immune System Works for details on T cells). Once infected, the T-helper cell turns into a HIV-replicating cell. T-helper cells play a vital role in the body's immune response. There are typically 1 million T-cells per one milliliter of blood. HIV will slowly reduce the number of T-cells until the person develops AIDS.
To understand how HIV infects the body, let's first look at the virus's basic structure. Here are the basic parts of the HIV virus:
Viral envelope - This is the outer coat of the virus. It is composed of two layers of fatty molecules, called lipids. Embedded in the viral envelope are proteins from the host cell. There are also about 72 copies of Env protein, which protrudes from the envelope surface. Env consists of a cap made of three or four molecules called glycoprotein (gp) 120, and a stem consisting of three to four gp41 molecules.
p17 protein - The HIV matrix protein that lies between the envelope and core
Viral core - Inside the envelope is the core, which contains 2,000 copies of the viral protein, p24. These proteins surround two single strands of HIV RNA, each containing a copy of the virus's nine genes. Three of these genes -- gag, pol and env -- contain information needed to make structural proteins for new virions.
HIV is a retrovirus, which means it has genes composed of ribonucleic acid (RNA) molecules. Like all viruses, HIV replicates inside host cells. It's considered a retrovirus because it uses an enzyme, reverse transcriptase, to convert RNA into DNA.
Once the HIV virus enters the body, it heads for the lymphoid tissues, where it finds T-helper cells. Let's look at how the HIV virus infects immune system cells and replicates:
Binding - The HIV attaches to the immune cell when the gp120 protein of the HIV virus 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 wall.
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 virus 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 cause the person's T-helper cell count to slowly dwindle. The lack of T-helper cells compromises the immune system. When a person's T-helper cell count drops below 200,000 cells per one milliliter of blood, he or she is considered to have AIDS. The development of AIDS takes about two to 15 years, but about half of all people with HIV will develop AIDS within 10 years after becoming infected, according to the CDC.
No one dies from AIDS or HIV specifically. Instead, an AIDS-infected person dies from infections, because his or her immune system has been dissipated. An AIDS patient could die from the common cold as easily as he or she could from cancer. The person's body cannot fight off the infection, and he or she eventually dies.