Attacking T cells

HIV targets cells of the immune system. Most important, it targets the T cells — white blood cells that fight infections either directly or indirectly. The type of T cell that is most susceptible to HIV infection is called the helper T cell. Helper T cells don't attack infection themselves, but they produce compounds that are involved in mediating the response of other T cells to the infection.

To do its dirty work, the virus looks around for something it can attach to, called a receptor. It finds the CD4 molecule on the T cell's surface and attaches to that.

Although the virus uses the CD4 molecule as a receptor, the molecule actually has some other purpose. Just as viruses hijack cells for their own purposes, HIV hijacks the CD4 molecule and uses it as an attachment point.

After attaching to the CD4 receptor, HIV attaches to a second receptor, called a co-receptor. HIV can use several co-receptors, but two are especially important in understanding the disease's progression:

i CCR5: This receptor is important in the early stages of infection. The form of HIV that attaches to the CCR5 receptor is called R5 HIV.

i CXCR4: This receptor is important later in the progression of the disease. The form of HIV that attaches to the CXCR4 receptor is called X4 HIV.

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