CD4 cells are an important part of the immune system. A CD4 count indicates how strong the immune system is, more specifically, how strong the immune system is to fight off infections. HIV affects the immune system by destroying CD4 cells and if a CD4 count drops too low, the risk of serious opportunistic infections increases. However, CD4 count recovery is possible with ART, meaning CD4 levels can go back to higher levels to keep people healthy.