that use only CXCR4 are termed X4, and those that use both, X4R5. However, the use of co-receptors alone does not explain viral tropism, as not all R5 viruses are able to use CCR5 on macrophages for a productive infection and HIV can also infect a subtype of myeloid dendritic cells, which probably constitute a reservoir that maintains infection when CD4⁺ T cell numbers have declined to extremely low levels.

Some people are resistant