Your immune system has different kinds of cells that work together to fight infections. CD4 cells are one kind of immune cell. These are often called T-cells. they are like little "factories" that make good things to tell your body how to fight germs and infections. These cells are found in your blood, lymph nodes and other places in your body. After HIV enters the body, it uses the CD4 cells to make more virus. At first, the body is able to make enough CD4 cells to keep up with those lost due to HIV. Over time, as the virus copies itself over and over, there are fewer and fewer CD4 cells. Eventually, the immune system is weakened and the body is less able to fight germs. Your CD4 count is important because it tells your doctor how strong your immune system is. Your body is like a city that needs lots of thing to run. Your cells are like factories that make things your body needs to live, like energy.