Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
HIV carries on its surface "spikes" which are the glycoprotein known as gp120. This gp120 binds with CD4, a protein which happens to protrude from various types of human cell. A gp120 sticking out of a HIV virus particle connects with a CD4 sticking out of a cell like two pieces of jigsaw joining together, and once the virus has attached to a cell in this way, in can go on to the next stage and merge with the host cell.
One of the main targets of HIV is also a very important type of cell for the body's immune response. CD4 lymphocytes">lymphocytes">T lymphocytes">lymphocytes ("T helper cells") come from the thymus and diffuse through the body. When they come into contact with foreign material such as virus particles, they start to multiply and to send out chemical signals which alert other cells of the immune system to attack the intruder.
There are other types of cell which carry CD4 on their surface - such as macrophages and some natural killer cells. Helper T cells are important, though: not only are they HIV's number one target, but since they're co-ordinators of the immune system, the disruption of their activity can have drastic effects on the infected person's response to other infections.
CD4 isn't the only important molecule on a cell's surface in terms of binding. Every HIV particle will bind primarily to CD4, but will not successfully complete the binding/fusion stages unless a coreceptor is present on the cell surface in addition to CD4. For T cells the coreceptor can be CXCR4 (fusin) or R5">CXCR5. For macrophages it is R5">CCR5.
Newsflash: Recent research has found that another pair of molecules is also important in the binding process. Apparently cyclophilin A is another protein which pokes out of particles of HIV, and it sticks to heparan molecules on the surface of cells. This seems to be at least as important as the gp120-CD4 binding...