CD4 and HIV infectious disease: However, in HIV infection, excessive immune activation poses a constant challenge for CD4+ and CD8+ memory T cells, leading to immune exhaustion and the loss of the ability to respond to HIV antigens, evidenced by the expression of markers such as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1 (PD-1), and immune dysfunction [16,17,18].