CD4 cells are white blood cells of the immune system that respond to infectious agents, such as viruses. Measuring CD4 cells helps determine how the immune system is responding to the HIV-1 infection. The relationship between HIV-1 viral load and CD4 counts is sometimes compared to a train heading for a crash. The crash is the point at which an infected person might become ill. Viral load shows how fast the train is going: slow, medium or high speed. The CD4 count shows how many miles of track are left before the crash!