HIV destroys CD4 T lymphocytes (helper T cells). If CD4 levels are low, this may indicate that HIV has progressed into AIDS.

Helper T cells are crucial for immune system function and activate after encountering antigens from disease-causing microorganisms. Antigens are biological markers that identify microorganisms such as bacteria and viruses.

When a CD4 count falls below a certain level, a person receives an AIDS diagnosis. The treatment a healthcare professional suggests depends on how low the CD4 count is.

Keep reading to learn more about T cells and their function and the link between T cell level and HIV and AIDS.

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T cells grow from stem cells in the bone marrow. They are a type of white blood cell. There are two main types of T cells: helper T cells and killer T cells. Ultimately, it is the killer T cells that attack and kill cells that pathogens have infected.

Helper T cells

Macrophages are another type of white blood cell. They consume disease-causing microorganisms, or pathogens, then present fragments of their antigens to helper T cells. When a helper T cell binds to the antigen fragment that it recognizes, it activates and alerts other white blood cells to the pathogen.

Helper T cells have CD4 proteins on their cell surface, which help them bind to antigen fragments. Because HIV destroys helper T cells, healthcare professionals use a CD4 count to measure CD4 levels and HIV progression.

Killer T cells

After receiving the alert, killer T cells seek out and destroy the pathogen (virus, bacteria, or disease-causing microorganisms). Other white blood cells, such as B lymphocytes, will also activate and produce antibodies in order to protect against the threat.

Learn more about T cells here.

HIV enters its genetic information into helper T cells to make copies of itself. When this happens, the helper T cells die. This severely disrupts the immune response. Low levels of helper T cells mean killer T cells and other white blood cells do not receive as much information about pathogens in the body. As a result, disease-causing bacteria and viruses multiply with minimal detection.

When the amount of helper T cells falls below 200 cells/mm3 (cells per cubic millimeter), a person may receive an AIDS diagnosis. But healthcare professionals will also take into account other variables such as overall white blood cell count and the percentage of lymphocytes.

AIDS is the most severe stage of HIV. When a person receives an AIDS diagnosis, their immune system is severely compromised, and they are at risk for opportunistic illnesses. The survival rate without treatment at this stage is typically 3 years.

CD4 T cells are helper T cells. They express, or manifest, a CD4 protein on their cell surface that helps them bind to antigen fragments. These antigen fragments belong to viruses, bacteria, and other microorganisms that could threaten a person’s health. Killer T cells express a CD8 protein on their cell surface.

When activated, helper T cells mobilize other white blood cells to initiate a full immune response. Killer T cells, for example, then seek out the pathogen and destroy it by releasing granzymes, which trigger cell death.

If someone’s helper T cells are below 200 cells/mm3, they will likely receive an AIDS diagnosis.

When a person has HIV, a healthcare professional will collect a blood sample and request a CD4 count. The CD4 count helps determine how many helper T cells a person has.

But when analyzing a CD4 count, healthcare professionals must take into account that:

  • CD4 levels could be lower in the morning
  • stress and fatigue may affect CD4 levels
  • corticosteroid levels could increase or decrease CD4 levels

All people whose helper T cells are below 200 cells/mm3 should receive a CD4 count every 3–6 months. If treatment is working, a person may only need a CD4 checkup every 6–12 months.

The CD4 count helps healthcare professionals monitor HIV progression and if the person is at risk for opportunistic illnesses.

When a healthcare professional wants a CD4 count, they take a blood sample from a person’s arm.

Side effects of drawing blood may include:

  • mild bruising
  • pain
  • dizziness
  • feeling faint

A healthcare professional will likely only need to draw a small amount of blood, so a person should not feel any significant side effects.

Usually, when someone receives an HIV diagnosis, they will start antiretroviral therapy (ART) as soon as possible.

If a person responds well to ART, their CD4 levels may increase by 100–150cells/mm3 after 1 year.

After analyzing a CD4 count, a healthcare professional can determine if the current care plan is working or if they need to introduce additional treatments.

As soon as CD4 levels drop below 200 cells/mm3, a healthcare professional may need to increase ART and administer other drugs to help bolster the immune system against opportunistic illnesses.

All people with HIV should receive a CD4 count every 3–6 months if their CD4 levels are below 200 cells/mm3, as this indicates a progression to AIDS. If the treatment is working and the CD4 count is stable, a person may only need a checkup every 6–12 months.

If a person receives an HIV diagnosis in time and starts ART promptly, it is unlikely their condition will progress to AIDS.

Taking ART not only keeps the volume of helper T cells high but also decreases the viral load (the amount of virus in the body).

If someone’s viral load decreases, it may reach an undetectable level. This means if a person keeps up the treatment for their condition, the virus cannot transmit to anyone through sex. Having an undetectable viral load also reduces HIV transmission during birth.

A healthcare professional requests a CD4 count to monitor helper T cell levels. When a person’s CD4 levels drop below 200 cells/mm3, the healthcare professional may diagnose that person with AIDS. If someone begins ART promptly after receiving an HIV diagnosis, their condition may never progress to AIDS.

T cells include two main types: helper T cells and killer T cells. Helper T cells express a CD4 protein on their cell surface that helps them bind to antigen fragments. These antigen fragments belong to disease-causing viruses and bacteria. After binding, the helper T cells signal other white blood cells to destroy the pathogen. Killer T cells are another type of T cell that break down pathogens by releasing granzymes that trigger cell death.