CD4 cells or T-cells are a type of white blood cells that play a major role in protecting your body from infection. They send signals to activate your body’s immune response when they detect “intruders,” like viruses or bacteria. Once a person is infected with HIV, the virus begins to attack and destroy the CD4 cells of the person’s immune system. HIV uses the machinery of the CD4 cells to multiply (make copies of itself) and spread throughout the body. This process is called the HIV life cycle.