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Doctor Decoded: What Are T Cells, Exactly?

Find out what T cells are, how they protect your health as a component of the immune system, and the role they play in different diseases.

Lauren Smith, MAPreeti Parikh, MD
Written by Lauren Smith, MA | Reviewed by Preeti Parikh, MD
Published on March 9, 2021

The immune system is complex and has a number of components. The one that probably gets the most attention is the T cell. You’ve likely heard of T cells, but what are they, exactly?

Getting to Know the White Blood Cells

In simplest terms, T cells are a subtype of lymphocyte, which is a type of white blood cell.

White blood cells are part of your body’s immune system, and they help fight off infections and diseases in various ways. There are three main categories of white blood cells:

  • Granulocytes

  • Monocytes

  • Lymphocytes

Lymphocytes are types of white blood cells that develop in the bone marrow. You can find them in your blood and lymph tissue (which is how they earned their name). There are two key types of lymphocytes:

  • B cell lymphocytes, or “B cells”

  • T cell lymphocytes, or “T cells”

B cells help make antibodies. These help fight off diseases. Plus, antibodies tend to “stick around” after a disease has passed, and this helps provide long-term immunity or protection from getting the illness again. (Learn more about antibodies here.)

T cells help control your immune response. They can also help kill cancer cells. Having healthy levels of them in your blood is important for being able to identify and attack threats to your health.

The Role of T Cells in Diseases

One of the reasons these cells get a lot of attention is due to their role in immunodeficiency. This is when someone’s immune system is weak or damaged, and this makes them more vulnerable to infections.

There are many causes of immunodeficiency. One of them is when T cells either don’t function well or are at abnormal levels. For example, infection with the human immunodeficiency virus (HIV) may lead to reduced levels of these special white blood cells. Without treatment, people with HIV are more prone to infections, such as tuberculosis. In fact, one of the methods of diagnosing HIV is with a T-cell count, which measures the number of T cells in the blood.

Next time you’re sick with the common cold, you can thank your T cells for silently getting to work and fighting off the sneaky virus. A strong and healthy immune system means that your common cold is no big deal, and you’ll likely be back to good health in no time.

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