Some factors contribute to the decreased susceptibility to HIV-1 infection observed in EUs, including genetic homozygosity for the deletion of 32 base pairs of the CCR5 chemokine receptor5, antiviral innate immunity factors6, secretion of chemokines and cytokines, the activity of natural killer cells4, IgA or IgG3 antibody production against envelope proteins7, 8, responses of T cells that are specific for HIV-19, cytotoxic responses of polyfunctional CD8 T cells10 and low-level activation of CD4 T cells11. Here, CD4 is linked to HIV-1 infection.