What Are CD4 T Helper Cells and Why Are They Important?

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CD4 T helper cells are a type of white blood cell called a lymphocyte that serves an important role in your immune system. These cells help defend the body against infection by directing other "killer" cells to neutralize foreign agents.

Without CD4 T helper cells, the immune system cannot recognize and coordinate a defensive response to diseases like HIV. When these cells are significantly depleted due to disease or other causes, the immune system can become "blind" to foreign agents and unable to defend itself. This is when a person is said to be immunocompromised.

This article explains what CD4 T cells are, their role in HIV infection, and how they are involved in the diagnosis and treatment of HIV.

Artist's depiction of a CD4+ lymphocyte amid red blood cells
Sciepro / Getty Images

What Are CD4 Helper T Cells?

T cells are a subset of lymphocytes that play different roles in the immune system. They are called T cells because they mature in the thymus gland after being produced by the bone marrow. Their counterpart, called B cells, are also produced in the bone marrow but mature in the spleen.

During maturation, certain T cells will develop a protein on their surface called CD4. These proteins act as receptors, binding to any agent it considers foreign and "recognizing" the agent by proteins on its surface (called antigens).

As each infectious agent has its own unique antigen, CD4 T cells are able to differentiate agents and trigger an immune response if an agent ever returns.

Types of CD4 T Cells

There are several types of CD4 T cells that serve different functions. They are commonly divided into conventional T helper (Th) cells and regulatory T (Treg) cells.

There are three types of T helper cells that serve specific functions:

  • Th1: These helper cells direct the innate immune response, meaning the generalized frontline attack to anything the body considers abnormal.
  • Th2: These helper cells direct the adaptive immune response, meaning the targeted immune defense against a specific infectious agent.
  • Th17: These helper cells are tasked with releasing an inflammatory chemical called interleukin 17 that helps neutralize certain viruses and fungi.

Treg cells are tasked with limiting the potentially damaging effects of Th cells. For example, interleukin 17 produced by Th17 can damage healthy cells if produced in excess. Treg cells prevent this from happening.

There are also specific CD4 T cells that "remember" past infections and launch a defensive response should the foreign attacker return. These are called memory CD4 T cells.

Role of CD4 T Cells in HIV Infection

CD4 T cells help fight HIV. Ironically, they are the same cells that HIV targets for infection.

During the initial acute stage of HIV, CD4 T cells can bring the infection under control because their numbers are high. But they don't fully clear the infection. Instead, the virus will go into hiding, embedding itself in tissues where it can remain undetected by the immune system for years.

In the meantime, the viruses remaining in circulation will "turn the tables" on the immune system by binding to CD4 T cells and inserting their genetic material in them. This effectively "reprograms" the infected cell, turning it into an HIV-producing factory.

After producing thousands of new viruses, the infected cell eventually dies.

Over the course of years, as more and more CD4 T cells are killed, the immune system gradually loses its ability to fight off otherwise harmless infections. These so-called opportunistic infections are what account for the majority of illnesses in people with advanced HIV.

(An example is a simple case of oral thrush that can spread to the lungs, bloodstream, and distant organs in people who are immunocompromised.)

When enough CD4 T cells are killed and the immune system is fully compromised, a person is said to have AIDS.

Does Everyone With HIV Get AIDS?

With rare exceptions, anyone with HIV who remains untreated will get AIDS. If treated with antiretroviral drugs, a person can today expect to live a normal to near-normal life expectancy.

Diagnostic Value of CD4 T Cells

By measuring the number of CD4 cells circulating in your blood, a healthcare provider can determine the relative strength of your immune system if you have HIV. This is done with a simple blood test called a CD4 count.

The CD4 counts measure the number of CD4 T cells in a cubic millimeter of blood (cells/mm3). In most healthy people, the CD4 count is between 500 and 1,500 cells/mm3.

The results of the CD4 count are broadly interpreted as follows:

  • Above 500 cells/mm3: Normal
  • 350-400 cells/mm3: Immunosuppression
  • 200-349 cell/mm3: Advanced HIV
  • Under 200 cells/mm3: Immunocompromised/AIDS

The CD4 count only describes the relative strength of your immune response. This is because it is possible to get a severe HIV-related infection, like tuberculosis, with a high CD4 count. It is also possible to have a low CD4 count and not get a severe HIV-related infection until your numbers drop below 100 or even 50 cells/mm3.

Can Your CD4 Count Return to Normal?

With early treatment, a person with HIV can often rebuild their immune system and return to a normal CD4 count. This is more difficult if treatment is delayed. If treatment is started when the CD4 count is very low (under 100), the numbers may improve but never get above 500.

Role of CD4 T Cells in HIV Treatment

In the past, treatment for HIV was delayed until the CD4 dropped to a certain level. This was due in part to the toxicity of early antiretroviral drugs as well as the speed at which drug resistance developed. These problems have been largely overcome.

Today, treatment of HIV starts immediately upon diagnosis. Antiretroviral drugs work by blocking stages in the virus' life cycle, preventing the virus from making copies of itself. The drugs don't kill the virus but reduce its numbers to undetectable levels where it can do the body little harm.

As such, the CD4 count plays no role in the initiation of treatment or the selection of drugs best suited for your specific HIV type.

Where it does play a part is in the initiation of prophylactic drugs used to prevent opportunistic infections. These antibiotic, antiviral, antifungal, or antiparasitic drugs are taken on a daily basis to prevent bacterial, viral, fungal, and parasitic opportunistic infection.

Examples include:

Summary

CD4 helper T cells are a type of white blood cell that coordinates the immune response. It can help fight many infections but is ironically the target of HIV infection. As HIV progressively depletes these cells, the body becomes less able to defend itself against otherwise harmless infections.

The CD4 count is a blood test used to measure CD4 T cells and the relative strength of your immune system. With early treatment, a person with HIV may return to a normal CD4 count of 500 or above. If left untreated, HIV will almost invariably progress to AIDS.

7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Mark Cichocki, RN
Mark Cichocki, RN, is an HIV/AIDS nurse educator at the University of Michigan Health System for more than 20 years.