T-cell Test

What are T-Cells?

A T-cell is a type of lymphocyte. Okay, so what’s a lymphocyte? Lymphocytes are a type of white blood cell. About 15 to 40 percent of your white blood cells are lymphocytes. And they are some of the most important cells in your immune system—protecting you from viral infections; helping other cells fight bacterial and fungal infections; producing antibodies; fighting cancers; and coordinating the activities of other cells in the immune system.

The two main types of lymphocytes are B-cells and T-cells. B-cells are created and mature in your bone marrow, while T-cells are created in bone marrow, but mature in your thymus gland (T for thymus). B-cells produce antibodies. Antibodies help the body destroy abnormal cells and infective organisms such as bacteria, viruses, and fungi.

T-cells are divided into three groups:

	Helper T-Cells (also called T4 or CD4+ cells) help other cells destroy infective organisms.
	Suppressor T-Cells (also called T8 or CD8+ cells) suppress the activity of other lymphocytes so they don’t destroy normal tissue.
	Killer T-Cells (also called cytotoxic T lymphocytes, or CTLs, and are another kind of T8 or CD8+ cell) recognize and destroy abnornal or infected cells.

What is a T4 Cell Count?

T4 cells. CD4+ cells. T-helper cells. No matter what you call them, these cells are important to know about if you are HIV-positive. (Note: whenever we generically refer to “T cells” on other pages of this web site, we are specifically referring to T4 cells). Knowing how many T4 cells you have—which is determined by blood tests ordered by your doctor—can tell you how healthy your immune system is and how well it is holding up in the fight against HIV. Your T4 cell count will also be helpful in figuring out when to start anti-HIV therapy and whether or not you should start taking medications to prevent AIDS-related infections.

T4 cells are responsible for signaling other immune system cells to fight an infection in the body. They are also the prime target of HIV, which can cause the number of these cells to decrease over time. Too few T4 cells means that the immune system will no longer functions like it is supposed to.

The normal T4 count is somewhere between 500 and 1500 cells per cubic millimeter of blood (a drop, more or less). In the absence of anti-HIV treatment, the T4 cell count decreases, on average, about 50 to 100 cells each year. AIDS-related diseases (opportunistic infections) such as Pneumocystis carinii pneumonia (PCP) can occur if your T4 count falls below 200. And a large number of other infections can occur if it drops below 50 to 100 cells. Because of this, drugs to prevent these infections (prophylactic treatment) are started once the T4 cell count falls below certain levels, such as 200 in the case of PCP.

Used in combination with viral load testing, your T4 cell count will also help you figure out when to start anti-HIV therapies. Most experts agree that anti-HIV therapy should be started when the T4 count is 350 or lower. There is considerable debate about whether to start therapy before then. For a complete review of this question of when to start treatment, click on the following lesson link:

What is the T4 Percentage?

If you look at your lab report, you will also see something called your CD4+ percentage (%). This is an important number for you and your doctor to know. In healthy adults, the number of T4 cells make up between 32% and 68% of the total number of lymphocytes—a large group of white blood cells that include T4 cells, T8 cells (see below), and B-cells. In fact, the lab uses the T4 percentage to determine the number of T4 cells in a sample of blood.

The T4 percentage is sometimes a more reliable measurement than the T4 count because it tends to vary less between measurements. For example, one person’s T4 count may vary between 200 and 300 over a several month period while their T4 percentage remains constant at, say, 21%. Provided that the T4 percentage stays at 21% or higher, the immune system still appears to be functioning properly, regardless of what the T4 count is. At the same time, a T4 percentage at or below 13%—regardless of what the actual T4 count is—usually means that the immune system is damaged and that it is time to begin prophylactic treatment (drugs to prevent diseases) for opportunistic infections like PCP.

What is a T8 Cell Count, and the T-cell Ratio?

T8 cells, also called CD8+ or Suppressor cells, play a major role in fighting infections such as HIV. A healthy adult usually has between 150 and 1,000 T8 cells per cubic millimeter of blood. Unlike T4 cells, people living with HIV tend to have higher-than-average T8 cell counts. Unfortunately, nobody fully understands the reasons for this. Therefore, this test result is rarely used in making treatment decisions.

Lab reports may also list the T-cell (CD4+/CD8+) ratio, which is the number of T4 cells divided by the number of T8 cells. Since the T4 count is usually lower than normal in people living with HIV, and the T8 count is usually higher, the ratio is usually low. A normal ratio is usually between 0.9 and 6.0. Like the T8 cell count, nobody really knows what this low number means. However, most experts agree that once anti-HIV therapy is started, an increase in the T-cell ratio (i.e. a rising T4 count and a falling T8 count) is a telltale sign that drug treatment is working.

What Does a T-cell Test Look Like?

You will usually find your T-cell counts and percentages listed under “Lymph Subset” or “T-cell Panel” in your blood lab reports, which refers to a listing of your various lymphocyte cells (CD3+, CD4+, and CD8+). See below for what a typical T-cell test looks like. Click on a test name in the left column to view an explanation of its significance & meaning. (note: to see and/or print a complete list of test name explanations, click here.)