The HIV Life Cycle
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.
Next, we'll see what happens when the HIV virus enters the body.