T-cells are a type of white blood cell called lymphocytes. They help your immune system fight germs and protect you from disease. There are two main types. Cytotoxic T-cells destroy infected cells. Helper T-cells send signals that direct other immune cells to fight infection.


What are T-cells (T lymphocytes)?

T-cells are a type of white blood cell called lymphocytes. They’re also called T lymphocytes. Lymphocytes play an essential role in your immune system. Your immune system fights infection-causing pathogens (viruses, bacteria, fungi and parasites) and harmful cells, like cancer cells.

Your lymphocytes include T-cells and B-cells. Both types are part of your body’s defense. B-cells make proteins called antibodies to fight pathogens. T-cells protect you by destroying harmful pathogens and by sending signals that help control your immune system’s response to threats.

What are the different types of T-cells?

There are two main types of T-cells:

  • Cytotoxic T-cells: Cytotoxic T-cells are also called CD8+ cells because they have a CD8 receptor on their membranes. These cells get their name from “cyto,” which means cell, and “toxic,” which means poisonous or harmful. Cytotoxic T-cells kill cells infected with viruses and bacteria, and they also destroy tumor cells.
  • Helper T-cells: Helper T-cells are also called CD4+ cells because they have a CD4 receptor on their membranes. Unlike cytotoxic T-cells, helper T-cells don’t kill cells directly. Instead, they send signals that tell other cells in your immune system how to coordinate an attack against invaders. Helper T-cells signal cytotoxic T-cells, B-cells and another type of white blood cell called a macrophage.

Although they’re not considered one of the main T-cell types, regulatory T-cells (suppressor cells) play an essential role in your immune system. These cells reduce the activity of other T-cells when necessary. They can prevent T-cells from attacking your body’s healthy cells.


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What do T-cells do?

T-cells are key fighters in what’s known as your adaptive immune system. Think of your adaptive immune system as a specialized smart system that’s constantly monitoring for threats. Once it detects an intruder, your adaptive immune system builds a customized defense to fight it.

Each T-cell is unique in that it’s designed to fight only one type of intruder. Once your immune system identifies the threat, it locates the specific T-cell designed to defeat it and recruits that T-cell for battle. The T-cell copies itself, making more T-cells to defeat the intruder. These T-cells that join the fight are called effector cells. When your immune system is working properly, these effector T-cells destroy the threat, helping rid you of infection and disease.

Your T-cells continue to protect you even after the intruder’s gone. Some of your T-cells become memory cells instead of effector cells. Unlike effector T-cells, memory T-cells aren’t fighters. Instead, they remember the intruder so that if it returns, your immune system recognizes it and quickly mounts a defense.

How do T-cells work in the immune system?

T-cells work once they’re activated. Several steps have to happen before a T-cell activation:

  1. A cell called an antigen-presenting cell (APC) locates evidence of the intruder and attaches it to a structure called major histocompatibility complex (MHC). This step is important because T-cells can’t recognize evidence of an intruder unless it’s attached to MHC.
  2. The T-cell binds to the MHC. There are two types of MHC. One fits each type of T-cell. The CD8 receptor on a cytotoxic T-cell can only bind to MHC-1. The CD4 receptor on a helper T-cell can only bind to MHC-II.
  3. Once the T-cell binds with all the matching parts, it activates. The binding is important because it ensures that the T-cell is the right one to fight the intruder.

An activated cytotoxic T-cell kills infected cells or cancer cells. An activated helper T-cell sends signals that tell other immune cells what actions to take to fight the intruder.


Where are T-cells located?

T-cells exist in different places depending on the point in the cell cycle. T-cells start in your bone marrow, mature in your thymus and eventually relocate to your lymph tissue or bloodstream.

  1. Bone marrow: T-cells start in the spongy tissue inside your bone called marrow. Like all blood cells, they start as hematopoietic stem cells. These cells have the potential to develop into any type of blood cell.
  2. Thymus: T-cells move to an organ called your thymus (located in your upper mid-chest) to mature. At this stage, the immature T-cells are called thymocytes. Your thymus is like boot camp for T-cells. Once inside, T-cells go through testing to be sure they can bind correctly to MHC and won’t attack your body’s healthy cells. They also receive the right receptor, either CD4 (helper T-cells) or CD 8 (cytotoxic T-cells). Only T-cells that pass all these tests go out into your body.
  3. Lymph tissue and bloodstream: Fully mature T-cells travel to tissue and organs in your lymph system, like your spleen, tonsils and lymph nodes. They may also circulate in your bloodstream. T-cells remain on standby in your body until you need them to protect you.

Your thymus is much larger when you’re a child and gets smaller as you age. Related, your thymus starts releasing significantly fewer T-cells starting at around age 20. Your supply of diverse T-cells depends more on your body’s ability to make copies of the T-cells you already have.


Conditions and Disorders

What are the common conditions and disorders that affect T-cells?

Several types of autoimmune diseases and immunodeficiency disorders can affect your T-cells. With autoimmune diseases, your immune system malfunctions and attacks your healthy cells. Immunodeficiency disorders may be inherited or acquired, but they involve having a weakened immune system.

Conditions that can affect your T-cells include:

  • Acute lymphocytic leukemia: A type of cancer that starts in your blood and bone marrow.
  • Adult Hodgkin lymphoma: A group of blood cancers that start in your lymphatic system.
  • T-cell lymphomas: A group of blood cancers that start in your T-cells and can affect different tissues, most commonly your skin, but also your lymph nodes and subcutaneous tissue.
  • Chronic T-cell leukemia (T-cell prolymphocytic leukemia): A blood cancer that starts in your T-cells that can affect your bone marrow, blood and lymph nodes.
  • DiGeorge syndrome: A genetic disorder that can prevent your body from making enough healthy T-cells.
  • HIV: A virus that attacks your white blood cells (especially your CD4+ T-cells) and potentially leads to AIDS.
  • Job syndrome: A rare immune system disorder that causes repeat infections.
  • Severe combined immunodeficiency (SCID): A group of rare genetic disorders that involves a weakened immune system resulting from problems with T-cells and other lymphocytes.
  • Thymic aplasia: A condition in which you’re born with an underdeveloped thymus.
  • Wiskott-Aldrich syndrome: A rare genetic condition that involves immune system issues, including atypical white blood cells.

What is the normal range of T-cells?

What’s considered a normal range for T-cells can vary depending on the lab that’s counting your T-cells. Generally, a CD4 count (helper T-cells) that’s between 500 to 1,200 cells/mm3 is considered normal if you’re a healthy adult. A CD8 count (cytotoxic T-cells) between 150 to 1,000 cells per cubic millimeter is considered normal.

Your healthcare provider can explain what a normal result looks like for you depending on your condition and general health.

What does it mean if your T-cells are low?

Low T-cells can signal a range of problems affecting your immune system. Low T-cells may be a sign of a condition that you’re born with that prevents your body from making enough T-cells. Your body may make fewer T-cells in response to a condition you acquire, like HIV. Or low T-cells may be a side effect of certain treatments or medications.

Regardless, not having enough T-cells makes it harder for your body to protect you from your body’s invaders. Having too few T-cells can even be life-threatening.

What does it mean if your T-cells are high?

It’s much less common to have too many T-cells. High T-cells may mean that your body has ramped up production of T-cells to fight an infection. High T-cells may also be a sign of certain cancers.


What are the common tests to check the health of my T-cells?

Your provider may order a T-cell count (also called a CD4 count) if they suspect there’s an issue with your immune system. Or they may order another test called a CD4 to CD8 ratio T-cell test to see how many helper T-cells you have in relation to cytotoxic T-cells.

T-cell counts are especially useful if you’re HIV-positive. They allow your healthcare provider to monitor your immune system’s health and gauge how your treatment’s working.


How can I boost my T-cells?

Ask your healthcare provider about medications and therapies that can boost your T-cells. In the meantime, you can take steps to benefit your immune system. Sometimes, the best way to protect your T-cells is to prevent yourself from having to rely too much on them. This means steering clear of germs and taking care of yourself.

You can:

  • Eat a well-balanced diet.
  • Stay up-to-date on all vaccines.
  • Avoid alcohol, or drink it in moderation.
  • Don’t smoke or vape and quit if you do.
  • Sleep at least seven to eight hours each night.
  • Engage in moderate exercise for at least 150 minutes a week.
  • Wear a mask in indoor areas, especially areas with poor ventilation.
  • Wash your hands frequently with soap and water or use hand sanitizer.

Additional Common Questions

What is the difference between T-cells and antibodies?

Both T-cells and antibodies protect you from pathogens, but they play different roles in your immune system. B-cells are the other type of white blood cell (lymphocytes). It’s B-cells (not T-cells) that make antibodies, a specific type of protein that kills harmful invaders. While B-cells send antibodies to kill harmful cells, cytotoxic T-cells kill harmful cells directly.

A note from Cleveland Clinic

T-cells are essential to both your immune system and your overall health. They play such an important role in protecting you from germs that you couldn’t survive without them. Ask your healthcare provider about how often you should be monitored if your T-cell count is low. Take all medications as directed. In the meantime, take care of yourself by putting healthy habits into place to protect yourself from getting sick.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 01/17/2023.

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