CD4 and infection: HIV-1 has a particular tropism for cells of the immune system that express the receptor CD4 [2] together with CCR5 or CXCR4, which the virus uses as co-receptors for cell attachment and entry [3], thus allowing the infection of CD4+ T cells but also other immune cell populations such as dendritic cells, monocytes/macrophages, and microglia [4].