What Are CD4 Cells (T Cells)?
CD4 cells are a type of lymphocyte (white blood cell). They are an important part of the immune system. There are two main types of CD4 cells. T-4 or CD4 cells are "helper" cells that lead the attack against infections. T-8 or CD8 cells are "suppressor" cells that end an immune response. CD8 cells can also be "killers" that kill cancer cells and cells infected by viruses.
Researchers can tell the different kinds of CD4 cells apart thanks to specific proteins found on the surface of the cell. The T-4 is a cell with CD4 molecules on its surface. This type of T cell is also called "CD4 positive" or CD4+.
Why Are CD4 Cells Important in HIV?
When HIV infects humans, the cells it most often infects are CD4 cells and when these multiply to fight infections, they also make more copies of the HIV.
When someone has been infected with HIV for a long time, the number of CD4 cells (CD4 cell count) falls. This is a sign that the immune system has been weakened. The lower the CD4 cell count, the more likely the person will get sick.
There are millions of different families of CD4 cells. Each family is designed to fight a specific type of germ. When HIV reduces the number of CD4 cells, some of these families may disappear. You may lose the ability to fight some of the germs that these families were designed for. If that happens, you may develop an opportunistic disease.
What Factors Affect the CD4 Cell Count?
The CD4 cell value changes constantly. Test results can vary according to the time of day or according to how tired or stressed a person is. The best thing is to use the same laboratory and to have blood drawn at the same time of day every time you have a CD4 cell test.
Infections can have a huge impact on CD4 cell counts. When your body is fighting infections, the number of white blood cells (lymphocytes) increases. The CD4 and CD8 count also increases. Vaccinations can have the same effect. Wait two weeks after recovering from an infection or receiving a vaccination before having a CD4 cell test.
How Are Test Results Reported?
The number of cells per cubic millimeter of blood (mm3) is specified. There is no agreement regarding the normal average range of CD4 cells. The normal CD4+ count is between 500 and 1600 and the CD8 count is between 375 and 1100. CD4 cells can fall drastically in HIV-positive people and in some cases can reach zero.
The ratio of CD4 cells to CD8 cells is sometimes analyzed. This is calculated by dividing the CD4 value by the CD8 value. In healthy people, this ratio is between 0.9 and 1.9, which means there are 1 to 2 CD4 cells for every CD8 cell. In people with HIV, this ratio drops, which means there are more CD8 cells than CD4 cells.
Since the CD4 cell count can vary greatly, some health care providers prefer to analyze the percentage of CD4 cells. This percentage refers to the total number of lymphocytes. If your test indicates you have 34% of CD4 cells, this means that 34% of your lymphocytes are CD4 cells. The percentage is more stable than the number of CD4 cells. The normal range is between 20% and 40%. Any percentage below 14% indicates serious damage to the immune system. It is a sign of AIDS in people infected with HIV.
What Do the Numbers Indicate?
The meaning of the CD8+ cell count is not clear and is still being studied.
The CD4 cell count is a key measure for determining the health of the immune system. The greater the damage done by HIV, the lower this count will be. According to the Centers for Disease Control (CDC), anyone with less than 200 CD4 cells or a CD4 percentage of less than 14% is considered to have AIDS.
The CD4 cell count is used together with the viral load to estimate how long someone will remain healthy.
The CD4 cell count is also used to determine when to start treatment with drugs.
Starting antiretroviral treatment (ART)
When the CD4 count falls below 350, most health care providers recommend ART. Some recommend ART when the percentage reaches 15%, even if the CD4 cell count is high. Some more conservative health care providers wait until the CD4 cell count falls to 200 before starting treatment. A recent study showed that starting treatment with a CD4 percentage below 5% is linked to worse results.
Starting drugs to prevent opportunistic infections
Most health care providers prescribe drugs to prevent opportunistic diseases at the following CD4 levels:
- Less than 200: pneumocystis carinii pneumonia.
- Less than 100: toxoplasmosis and cryptococcosis.
- Less than 75: mycobacterium avium complex.
Since CD4 cells are a very important indicator of the strength of the immune system, official US treatment guidelines suggest getting tested every 3 to 4 months.
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