T cell

Tiny cells help keep us safe.

These cells are born in your bones. Then they move to a small part of your body. They grow there to become strong.

Some of these cells are like fighters. They can kill bad cells. Other cells are like helpers. They tell other cells to get ready.

One group helps the body know itself. They make sure the fighters do not attack you. This keeps your body healthy. These tiny cells work very hard every day.

T cells are special parts of your immune system. They help your body fight off germs.

These cells start in your bone marrow. Then they move to a gland called the thymus. This is why they are named T cells. Inside the thymus, they go through a big test. They must make a T cell receptor, or TCR. This is a tool on their surface. It helps them find and recognize threats.

Most cells do not pass the test. About 98% of them die during this time. The ones that survive become different types. Some are CD8+ killer T cells. These cells can kill cancer or virus cells.

Other cells are CD4+ helper T cells. They help by turning on other immune cells. There are also regulatory T cells. These cells act like a brake. They help the body tell the difference between germs and your own cells. This stops the body from attacking itself. As people get older, the thymus gets smaller. This means the body makes fewer new T cells.

T cells are a vital part of your immune system. They are a main part of the adaptive immune response. This means they help your body learn to fight specific threats. You can spot them because they have a T-cell receptor, or TCR, on their surface. This receptor acts like a tool to recognize different germs.

These cells follow a very specific path to grow up. They start as stem cells in your bone marrow. Some may even start in the fetal liver before birth. These cells turn into common lymphoid progenitors. Then, they travel through your blood to a gland called the thymus. This is where they get their name. Inside the thymus, they are called thymocytes.

Growing into a T cell is a hard job. The cells must build a working TCR using alpha and beta chains. They first test a beta chain with a mock alpha chain. Then they build a real alpha chain. The cells must pass two big tests called selection. First, they undergo positive selection in the thymic cortex. They must show they can recognize the body's MHC molecules.

After that, they face negative selection in the thymic medulla. This test makes sure the cells do not attack your own body. If a cell reacts too strongly to "self" antigens, it dies. This prevents an autoimmune response where the body attacks itself. Most cells do not pass these tests. In fact, about 98% of thymocytes die during this process. Only about 2% survive to become mature T cells.

There are different types of T cells with different jobs. CD8+ T cells are often called "killer" T cells. They can directly kill cancer cells or cells with viruses. CD4+ T cells are known as "helper" cells. They activate other cells like B cells to create a larger response. There are also regulatory T cells that act as a brake. These cells help the immune system tell the difference between germs and your own cells.

T cells, also known as T lymphocytes, are a central part of the adaptive immune response. These cells are essential for helping the body recognize and fight specific pathogens. You can distinguish them from other lymphocytes by the T-cell receptor, or TCR, located on their cell surface. This receptor allows the cell to detect specific patterns of invaders.

The journey of a T cell begins in the bone marrow. They originate from c-kit+Sca1+ hematopoietic stem cells. In some cases, they may originate in the fetal liver during embryonic development. These stem cells differentiate into multipotent progenitors, or MPPs. These progenitors can become either myeloid or lymphoid cells. Eventually, they become common lymphoid progenitors, or CLPs. These cells can only turn into T, B, or NK cells. The CLPs then migrate through the blood to the thymus gland to mature.

Once they arrive in the thymus, these immature cells are called thymocytes. Their development involves several complex stages. The earliest cells are called double-negative because they lack both CD4 and CD8 co-receptors. They start as early thymic progenitor (ETP) cells. As they divide, they become double-negative one (DN1) cells. They eventually progress through various DN stages. During this time, they express specific markers like CD2, CD5, and CD7. Eventually, they express both CD4 and CD8, making them double-positive cells.

A critical part of this maturation is building a functional T-cell receptor. The TCR consists of two main parts: the alpha chain and the beta chain. Each cell creates a unique TCR through random variation. This ensures the immune system can recognize many different types of pathogens. First, the thymocyte attempts to create a functional beta chain. It tests this chain against a mock alpha chain. If successful, the cell produces a pre-TCR. This signal allows the cell to move to the double-positive stage. There, it begins to rearrange the TCR alpha locus.

To become active, thymocytes must pass two rigorous tests called selection. Positive selection occurs in the thymic cortex over three to four days. Here, thymocytes are presented with self-antigens on MHC molecules. Only cells that interact well with MHC-I or MHC-II receive a survival signal. Cells that cannot interact strongly enough die from neglect. This ensures the cells have the correct MHC affinity. This process determines if a cell becomes a CD4+ helper cell or a CD8+ killer cell.

Next, the cells undergo negative selection in the thymic medulla. This process is vital for preventing autoimmune responses. In the medulla, cells are presented with self-antigens by medullary thymic epithelial cells, or mTECs. These mTECs must be AIRE+ to express these specific antigens. If a thymocyte binds too strongly to a self-antigen, it receives a signal to die. This prevents the immune system from attacking the body's own cells. Some of these cells are instead chosen to become regulatory T cells.

The selection process is extremely strict. About 98% of all thymocytes fail and die during development. Only about 2% survive to leave the thymus as mature naive T cells. As people age, the thymus begins to shrink by about 3% each year. This causes a fall in the production of new naive T cells. Consequently, older people rely more on the expansion of existing cells.

Mature T cells are grouped into subsets based on their specific functions. CD8+ T cells are known as killer or cytotoxic T cells. They can directly kill cancer cells or virus-infected cells. They also use signaling proteins called cytokines to recruit other cells. CD4+ T cells are known as helper T cells. They activate B cells and other cytotoxic T cells to create a larger response. Different subtypes, like T-helper1 or T-helper2, are defined by the cytokines they secrete.

Regulatory T cells, or suppressor T cells, provide a critical mechanism of tolerance. They help the immune system distinguish between invading cells and "self" cells. This prevents the immune system from reacting against the body's own tissues. However, cancer cells can sometimes co-opt these regulatory cells. If they do, the cancer can prevent the immune system from recognizing the tumor. This allows the tumor to grow without being attacked.