CD4 cell Test

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What are CD4 Cells?

A CD4 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 abnormal or infected cells.

The "c" and the "d" in CD4 stands for "cluster of differentiation," and refers to the cluster of proteins that make up a cell surface receptor. There are dozens of different types of clusters, but CD4s and CD8s are the ones we talk about most often.

What is a CD4 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. Knowing how many CD4 cells you have, which is determined by blood tests ordered by your doctor and refers to the number of cells in the small amount of blood being tested, can tell you how healthy your immune system is and how well it is holding up in the fight against HIV. Your CD4 cell count will also be helpful in figuring out when to start antiretroviral (ARV) therapy and whether or not you should start taking medications to prevent AIDS-related infections.

CD4 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 CD4 cells means that the immune system will no longer functions like it is supposed to.

To see a picture of a CD4 cell being infected by HIV, click here: HIV, AIDS virus, & T4 picture

The normal CD4 count is somewhere between 500 and 1500 cells per cubic millimeter of blood (a drop, more or less). In the absence of HIV treatment, the CD4 cell count decreases, on average, about 50 to 100 cells each year. AIDS-related diseases (opportunistic infections) such as Pneumocystis pneumonia (PCP) can occur if your CD4 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 CD4 cell count falls below certain levels, such as 200 in the case of PCP.

Your CD4 cell count will also help you figure out when to start ARV treatment. Most experts agree that ARV therapy should be started when the CD4 count is 500 or lower. For a complete review of this question of when to start treatment, click on the following lesson link:

Lesson When Should I Start Treatment, and What Should I Take First?

What is the CD4 Percentage?

If you look at your lab report, you will also see something called your CD4+ percentage (%). In healthy adults, the number of CD4 cells make up between 32% and 68% of the total number of lymphocytes, a large group of white blood cells that include CD4 cells, CD8 cells (see below), and B-cells. In fact, the lab uses the CD4 percentage to determine the number of CD4 cells in a sample of blood.

The CD4 percentage is sometimes a more reliable measurement than the CD4 count because it tends to vary less between measurements. For example, one person's CD4 count may vary between 200 and 300 over a several month period while their CD4 percentage remains constant at, say, 21%. Provided that the CD4 percentage stays at 21% or higher, the immune system still appears to be functioning properly, regardless of what the CD4 count is. At the same time, a CD4 percentage at or below 13%, regardless of what the actual CD4 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 CD8 Cell Count, and the T-cell Ratio?

CD8 cells, also called T8 cells, play a major role in fighting infections such as HIV. A healthy adult usually has between 150 and 1,000 CD8 cells per cubic millimeter of blood. Unlike CD4 cells, people living with HIV tend to have higher-than-average CD8 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 CD4 cells divided by the number of CD8 cells. Since the CD4 count is usually lower than normal in people living with HIV, and the CD8 count is usually higher, the ratio is usually low. A normal ratio is usually between 0.9 and 6.0. Like the CD8 cell count. Some experts think that the reversed ratio in people living with HIV is a kind of one-two punch by HIV. One the one hand, it contributes to T-cell death and turnover, eventually decreasing CD4 counts. On the other hand, because the virus keeps the immune system in a state of constant inflammation, this can cause CD8 cell numbers to be chronically inflated. However, most experts agree that once anti-HIV therapy is started, an increase in the T-cell ratio (i.e. a rising CD4 count and a falling CD8 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+) and other immune cells. This report is often called a complete blood count, or CBC, report. 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.)

Clinical Laboratory Report
  Patient Name Date Drawn Date Received Date of Report
    DOE, JOHN     12/20/13     12/20/13       12/22/13  
  Sex Age Client Name / Address I.D. Number Account Number
    M     31     MEDICAL CENTER
  ANYTOWN     US        10023
    78987654     12343  
  Ordering Physician   Time
    Specimen Number  
        918273     11:00    
  Patient I.D./Soc. Sec Number
625 - 2460
60 - 90
423 - 1724
CD3+/CD4+ (HELPER) %
23 .8    L
32 - 68
1290       H
140 - 958
53.9    H
10 - 36
0.44  L
0.90 - 6.00


Last Revised: May 04, 2014

This content is written by the POZ and AIDSmeds editorial team. For more information, please visit our "About Us" page.

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