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How does the loss of CD4 T cells lead to AIDS?

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AIDS Scanning electron micrograph of HIV-1 virions (green) budding from a cultured lymphocyte. Multiple round bumps on the cell surface represent sites of virion assembly and budding.

Helper T cells, also known as CD4+ T cells, are essential to the immune system because they activate other immune cells, including B cells that produce antibodies and cytotoxic T cells that destroy infected cells.12

HIV causes AIDS by specifically targeting and infecting these CD4+ cells. The virus uses the CD4 protein on the cell surface to gain entry, subsequently hijacking the cell's machinery to replicate. This process leads to the death of the infected helper T cells.1

As the virus continues to replicate, the rate of helper T cell destruction exceeds the body's ability to produce new ones, resulting in a progressive decline in the CD4 count. When this count drops significantly—typically to about 200 cells per microliter of blood—the immune system becomes severely compromised. This state of profound immune deficiency leaves the body unable to fight off pathogens, leading to the development of opportunistic infections and certain cancers that define the final stage of HIV infection, known as AIDS.134