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. (Note: whenever we generically refer to "T cells" on other pages of this web site, we are specifically referring to CD4 cells). 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. YourCD4 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 

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 anti-HIV treatment, the CD4 cell count decreases, on average, about 50 to 100 cells each year. AIDS-related diseases (opportunistic infections) such as Pneumocystis jiroveci 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.

Used in combination with viral load testing, 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 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 CD4 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 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.

Understanding Your Viral Load Test

The U.S. Department of Health and Human Services (DHHS)—the federal agency responsible for setting health-related policies in the United States—regularly updates and publishes HIV treatment guidelines to help HIV-positive patients and their health care providers determine when antiretroviral therapy should be started. Here is what the guidelines, revised in March 2012, recommend:

 

Health Status and CD4 Cell Count

Recommendation

•CD4 count less than 350

•History of an AIDS-defining illness (see our list of opportunistic infections), regardless of the CD4 cell count.

•Pregnant women, regardless of the CD4 cell count.


Antiretroviral therapy is recommended.

This is a strong recommendation, based on data from rigorous randomized, controlled clinical trials.

•Patients with CD4 counts between 350 and 500

•A diagnosis of HIV-associated nephropathy (kidney disease caused by HIV infection), regardless of the CD4 cell count.

•People coinfected with hepatitis B virus (HBV) and HIV, regardless of the CD4 cell count, when HBV treatment is recommended. The treatment regimen selected should include antiretrovirals active against HIV and HBV (Viread and/or Emtriva or Epivir, for example).


Antiretroviral therapy is recommended.

This is a strong recommendation, based on data from slightly less rigorous non-randomized clinical trials or observational cohort studies.

•Patients with more than 500 CD4 cells.


Antiretroviral therapy is recommended.

This is a moderate recommendation, based on expert opinion, not on the results of clinical trials or observational cohort studies.

Importantly, the guidelines panel stresses that the decision to start HIV therapy should be individualized. It is important for people living with HIV to first be able to commit to life-long HIV treatment, and be fully aware of both the importance of adherence and the risks and benefits of treatment. Treatment can be delayed, for example, if an HIV-positive person is dealing with issues that might limit the success of therapy, including acute medical problems, psychological issues (i.e., depression), or social issues (i.e., lack of housing). In other words, people with HIV and their health care providers will have to weigh their options carefully when considering earlier HIV treatment.