context: The CD4 count is a test that measures how many CD4 cells you have in your blood. These are a type of white blood cell, called T-cells, that move throughout your body to find and destroy bacteria, viruses, and other invading germs. Your test results help your doctor know how much damage has been done to your immune system and what's likely to happen next if antiretroviral treatment ( ART) is not initiated. All persons with HIV should be started on ART regardless of whether the CD4 count is high or low. The CD4 count should increase in response to effective ART. Keeping your CD4 count up with an effective ART can hold off symptoms and complications of HIV and help you live longer. In fact, studies have found that patients who adhere to regular treatments can achieve a life span similar to persons who have not been infected with HIV. Persons with very low CD4 counts may need to take drugs to prevent specific opportunistic infections ( OIs) in addition to taking their ART. Once the CD4 count increases in response to ART, it may be possible to stop taking these OI medications. HIV damages your immune system because it targets CD4 cells. The virus grabs on to the surface of a cell, gets inside, and becomes a part of it. When the infected CD4 cell dies, it releases more copies of HIV into he bloomstream. Those new bits of virus find and take over more CD4 cells, and the cycle continues. This leads to fewer and fewer HIV-free, working CD4 cells. HIV can destroy entire "families" of CD4 cells, and then the germs these cells fight have easy access to your body. The resulting illnesses are called opportunistic infections ( OIs) because they take advantage of your body's lack of defense. A normal CD4 count is from 500 to 1,400 cells per cubic millimeter of blood. CD4 counts decrease over time in persons who are not receiving ART. At levels below 200 cells per cubic millimeter, patients become susceptible to a wide variety of OIs, many of which can be fatal. The test results don't always match how well you're feeling though. Some people can have high CD4 counts and be doing poorly, for other reasons. Others can have low CD4 counts but feel well, with few complications. However, such patients are at risk of becoming very ill if they don't start HIV treatment. Anyone who is HIV-positive should take antiretroviral therapy ( ART) medications, regardless of their CD4 count and whether or not they have symptoms. When your treatment is working, your CD4 count should stay steady or go up. If your CD4 count keeps going down over several months despite adhering to ART, it is possible that your virus is developing resistance to the drugs that you are taking. This should be apparent from HIV viral load tests, which your doctor should be doing every few months. In this case, your doctor may want to change your ART drugs. Things other than the HIV virus can influence how high or low your CD4 count is, too. An infection like the flu, pneumonia, or a herpes simplex virus (including cold sores) can make your CD4 count go down for a while. Your CD4 count will go way down when you're having chemotherapy for cancer. To get the most accurate and helpful results for your CD4 count, try to: Use the same lab each time. Wait for at least a couple of weeks after you've been sick or gotten a shot before you get a test. Right after you're diagnosed, you should get a CD4 count for a "baseline measurement." That gives your doctor something to compare future test results to. It may also indicate the need for drugs to prevent specific OIs in addition to the drugs you are taking for HIV. A viral load test 2 to 8 weeks after you start or change treatment helps your doctor decide how well the ART is working. A CD4 test will indicate whether the immune system is improving in response to ART. Then you should typically get a CD4 test every 3 to 6 months, or as often as your doctor recommends, to see how well your immune system is doing. Persons with low CD4 counts who are taking drugs to prevent specific OIs in addition to their ART may be able to stop these OI drugs as their immune system responds to ART. Persons with CD4 counts above 500 who maintain viral suppression may not need further CD4 testing.