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Your immune system protects your body by fighting germs and infections. White blood cells are an important part of your immune system . HIV infects and destroys a type of white blood cell called a CD4 cell (sometimes called a T-cell).
As the immune system loses CD4 cells, it becomes weaker and is less able to fight off germs. When it loses a large number of CD4 cells, people living with HIV (HIV+) are at risk of getting AIDS-related opportunistic infections  (OIs), which are infections that can cause serious illness or death.
The number of CD4 cells you have gives a picture of the health of your immune system. A normal CD4 cell count is about 500 to 1,500 cells per cubic millimeter of blood (a cubic millimeter is a very small amount, about one drop). The number of CD4 cells a person has – their "CD4 count" – usually decreases as HIV disease gets worse. Your CD4 cell count can help your health care provider tell if your HIV disease is getting worse, and if the treatment you are taking is working well.
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Without HIV treatment, HIV infects and destroys more and more CD4 cells. As the CD4 count goes down, people living with HIV becomes more likely to develop OIs and cancers .
- CD4 count > 500: People with CD4 counts above 500 cells usually have a fairly normal immune system and are at low risk for opportunistic infections
- CD4 count < 200: People with CD4 counts below 200 cells are diagnosed as having AIDS and are at risk for developing serious opportunistic infections such as Pneumocystis pneumonia (PCP), Mycobacterium avium complex (MAC), and cytomegalovirus (CMV). If your CD4 count drops below 200, your health care provider will likely recommend that you take medications to prevent these infections.
There are several different organizations and institutions that make recommendations about when to start HIV treatment. The US Department of Health and Human Services (DHHS), WHO (World Health Organization, EACS (European AIDS Clinical Society), BHIVA (British HIV Association), and the IAS-USA (International AIDS Society USA) all recommend that HIV treatment be offered to all people living with HIV, regardless of their CD4 count. Researchers have shown that people living with HIV who start treatment earlier, while their CD4 counts are still high, have a much lower risk of illness and death.
The DHHS treatment guidelines state:
- HIV treatment is recommended for anyone who is living with HIV, no matter what their CD4 count
- HIV treatment is also strongly recommended if you are in one or more of the following situations, no matter what your CD4 count:
- The guidelines also point to several conditions that increase the need for treatment, such as rapidly declining CD4 count (more than 100 cells per year) or higher viral load (more than 100,000 copies)
- HIV drugs should be offered to people who are at risk of spreading HIV  to their sexual partners
Many people see their CD4 counts increase when they start effective HIV treatment . If the drugs succeed in slowing or stopping HIV, fewer new CD4 cells will be infected and your CD4 count may go up. However, your CD4 count can also go down again if you stop taking your HIV drugs correctly, or if your HIV becomes resistant  to the drugs. Along with your viral load , your CD4 count is a very valuable tool for monitoring your HIV infection and how well your HIV drugs are working.
The CD4 cell test is a simple blood test ordered by your health care provider. When you first start receiving care for HIV, you should get a "baseline" CD4 cell test. This baseline test gives a picture of your immune system when you first enter into care. Later tests can be compared against this first result to see how things are changing over time and with treatment.
It is important to get your CD4 count checked about every three to six months – or as often as your health care provider recommends. You will need more frequent CD4 cell tests if your count is low or falling, or if you are starting or changing treatment. If you have been on HIV drugs for over two years, are virally suppressed and have a CD4 count over 300, your provider may suggest that you only need to get your CD4 count checked once a year.
Many factors can affect your CD4 count, including the time of day, level of stress, your menstrual cycle, and infections such as the flu. If you get a result back that surprises you or your health care provider, he or she will probably want you to get a second test. The second test would confirm any unexpected results or show that the first test's results were random – the result of lab error or an unimportant occurrence. Try not to worry too much about a single abnormal test result; trends over time are usually more important.
In addition to your CD4 cell count, your health care provider will want to check your CD4 percentage. This number tells you what percent of your total white blood cells are CD4 cells. A normal CD4 cell percentage is about 30 to 60 percent.
The CD4 percentage is sometimes a more reliable measurement than the CD4 count because it tends to change less between measurements. While the CD4 percentage gives information about the health of your immune system, the CD4 count is the preferred measurement for assessing progression of HIV disease. Currently, treatment guidelines for adults are based on CD4 counts and not on CD4 percentage.
If you begin treatment when your CD4 count and percentage are low (CD4 count <200), it may take longer for your CD4 numbers to increase. However, if your viral load is below detectable levels, then you can be encouraged that the virus is not growing or spreading. Sometimes watching your CD4 cells and percentage increase can take lots of time and patience.
Because HIV infects and kills CD4 cells, CD4 counts usually drop as HIV disease gets worse. Taking effective combinations of HIV drugs may stop your CD4 count from dropping and improve your health.
Your CD4 count is an important indicator of the health of your immune system . Keeping track of the trends in your CD4 count can help you and your health care provider make decisions about starting and changing treatment. Getting regular CD4 cell tests—along with viral load tests and other blood tests to check for treatment side effects—is an important way to take charge of your health.