Types Of T Cells And Their Functions

Types Of T Cells And Their Functions – T cells are a type of immune cell that play an important role in protecting the body against specific threats, and rely on the detection of antigens: abnormal or unknown molecules that identify a potential threat. Some T cells circulate throughout the body, but most reside in the lymph nodes, waiting to be activated to respond to danger. When a threat is detected, T cells multiply into a T cell army that hunts down and kills the threatening cells.

There are three main types of T cells: cytotoxic T cells, helper T cells, and regulatory T cells. Cytotoxic T cells produce toxic substances to kill their targets. Helper T cells stimulate B cells to make antibodies against the targets and stimulate cytotoxic T cells to become active. Regulatory T cells suppress immune responses and keep the system in check to prevent overactive immune responses.

Types Of T Cells And Their Functions

Although all three types of T cells play an important role in an immune response, cytotoxic T cells are considered the warriors of the immune system. Although they are unable to directly attack foreign invaders (such as viruses and bacteria), they can attack and kill cells of the body that are infected or show signs of an abnormality. When cytotoxic T cells encounter the specific threat they are programmed to face, they release molecules that directly damage and kill the threat. T cell attacks are very powerful and are one of the best weapons the immune system has against threats in the body. T cells are often considered the key cells involved in the immune response against cancer, and in cancer immunotherapy, many treatment options aim to enhance T cell responses.

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T cells use structures on their surfaces, appropriately called T cell receptors (TCRs), to very specifically recognize abnormal or unknown molecules, called antigens, that may be present on invading or unhealthy target cells. Each T cell receptor recognizes one particular antigen. All receptors on an individual T cell are identical, but differ from the receptors on all other T cells. With millions of T cells in the body, these highly specific cells as a group can recognize virtually any molecule that enters the body.

When an individual’s immune system develops early in life, T cells begin to take shape. Early T cells encounter common or “normal” molecules in the body, and any T cells that recognize these common “self” molecules die, leaving only the T cells that respond to “other” molecules. This process essentially trains a person’s immune system so that it does not respond to normal molecules in the body, while leaving behind a huge army of T cells capable of recognizing “foreign” molecules, or molecules that are unusual. Some of these T cells circulate in blood and tissues, but most lurk in the lymph nodes until something comes along to activate them.

A wide variety of immune cells patrol the body, looking for signs of a threat. These cells can identify unknown patterns in their environment, including patterns that can identify invaders, tissue damage, or cancer. When a potential threat is detected, antigen-presenting cells such as dendritic cells “eat” cells or debris at the site of the threat, accumulating antigens. They can then break down what they’ve eaten and take with them harmless, representative pieces of the antigens as they migrate to nearby lymph nodes – small, bean-shaped organs located throughout the body.

T cells lurk in the lymph nodes, ready to launch an attack if evidence of a threat is presented to them. When antigen-presenting cells present the pieces of antigens they have collected as evidence of a potential threat, T cells with the T cell receptors corresponding to the specific antigens will become activated. This match between the antigen and the T cell receptor sends the first “go” signal for T cell activation. However, T cells must reach a certain threshold of ‘go’ signals before they are ready to launch an attack.

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Antigen recognition begins the process of T cell activation, but many signals are needed to fully bring the T cells into action. One important signal is provided directly by the antigen-presenting cells via surface molecules called CD80 and CD86. These molecules correspond to receptors called CD28 on the T cell surface and provide a “go” signal. When a T cell receives enough ‘go’ signals through the CD28 receptors, it becomes more activated and is ready to launch an attack.

While the CD28 receptor on T cells sends “go” signals, T cells also have built-in systems called “checkpoints” that essentially send “stop” signals to prevent T cells from overreacting. These ‘go’ and ‘stop’ signals balance each other and determine whether a T cell is activated, and to what extent the T cell is activated. One important “stop” signal is provided by a receptor called CTLA-4.

As the T cell becomes more activated, it increases production of CTLA-4, which is then present on the T cell surface alongside CD28. These two molecules are very similar and compete with each other for interactions with CD80 and CD86 on dendritic cells. While CD28 provides “go” signals, CTLA-4 provides “stop” signals. If the “stop” signals outweigh the “go” signals, T cell activation can be limited or prevented.

CTLA-4 plays an important role in preventing T cell responses against healthy cells from being activated or from T cell responses from becoming too activated and getting out of control. However, in the case of cancer immunotherapy, highly activated T cell responses may be desirable, and blocking CTLA-4 may be useful as a form of therapy.

B Cells Versus T Cells: What Are Lymphocytes?

Once a T cell becomes sufficiently activated, it will begin to multiply rapidly, forming an army of T cell clones – all identical and capable of recognizing the exact same target. These T cells then leave the lymph nodes and begin spreading throughout the body to seek out and attack the specific threat they recognize.

When cytotoxic T cells encounter the antigens they are programmed to target, they release molecules that directly damage and kill target cells. T cell attacks are very powerful and are often considered the best weapon against threats that have compromised the body’s cells.

Like cytotoxic T cells, helper T cells multiply, spread throughout the body, and hunt for their target molecules. However, helper T cells do not typically kill target cells directly, but rather release signals that help other immune cells, including cytotoxic T cells, B cells, and macrophages, carry out their immune functions. The support of helper T cells is essential for a strong immune response to any threat.

When regulatory T cells are activated, they multiply and spread in the same way as cytotoxic T cells and helper T cells, searching for their target molecules. However, their role is very different once they find their target. Instead of supporting immune responses, regulatory T cells help slow or suppress immune responses. This is important to prevent immune attacks from targeting healthy cells, getting out of control or lasting too long, as this could lead to unnecessary damage to the healthy tissue surrounding the site of the threatening cells.

Regulatory T Cells: Purpose, Function & Development

Cytotoxic T cells also have other built-in checkpoints that tell them to slow or stop the attack on a threat. One such checkpoint is the PD-1 receptor, which begins to appear on the surfaces of T cells after they are activated. PD-1 acts as a set of brakes on the T cell, which can be pressed by another molecule, PD-L1 (or in some cases PD-L2). Cells under attack by the immune system often respond by increasing the production of PD-L1, which then appears on the cell surface as a defense mechanism. When a T cell with PD-1 on its surface encounters a cell with PD-L1 on its surface, the brakes are applied and the T cell begins to slow down until it eventually becomes exhausted or dysfunctional and stops attacking.

The PD-1 checkpoint plays an important role in slowing and ultimately stopping an active T cell response once a threat has cleared from the body. However, cancer cells (and other cells that help the cancer survive and grow) are notorious for abusing this natural checkpoint and producing large amounts of PD-L1 as a defense mechanism. In cancer immunotherapy, blocking PD-1 or PD-L1 is a useful strategy to maintain strong T cell responses in the long term.

Once a threat is gone from the body, most T cells die and are cleared away on their own, but some turn into “memory” T cells that can survive in the long term. These cells remain on patrol in the body, and if the same threat is ever encountered again, these cells are already armed and ready to attack the threat and quickly restart a larger immune response.

T cells are often considered the key cells in immune responses against cancer, and many cancer immunotherapies target them

T And B Lymphocytes

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