T cells, which are also known as T lymphocytes, are among the key elements of our adaptive immune system. The roles of these T cells involve destroying the infected host cells directly, stimulating the other cells of the immune system, controlling the immune response as well as producing cytokines.
Precisely speaking, a T cell is a form of white blood cell (leukocyte) that is found in the human body. The name of this adaptive immune system element is actually an abbreviation of Thymus cell. It is the organ where this form of white blood cell develops. It is worth mentioning here that these cells are vital for preserving the immune system of the body. Moreover, they are vital in the process of combating any detrimental invading substance.
When any invading substance, including viruses and bacteria, enter an individual’s system, his/her body produces an immunological reaction. Specific cells with receptors are able to identify these invading substances and they can stimulate various different cells in our body to track and kill the detrimental invaders. People whose immune system is potent usually have lots of white blood cells (leukocytes) as well as T cells in their body. This is why people enduring autoimmune diseases or those suffering from cancer are subjected to frequent blood count tests to let their doctors know how effectual and helpful these cells are in their body in combating their ailments.
In fact, our body has many diverse kinds of T cells and each variety of these cells have specific functions. For instance, T helper cells have the ability to divide quickly and aid in the entire immune response of the body in attacking the harmful invaders. They do this by creating a useful protein known as cytokine. Another type of T cell, which is often known as memory cell, aids the body in combating repeat infections, thereby helping an individual to avoid suffering from an ailment more than once.
On the other hand, cytotoxic cells, a very helpful T cell, also come with a major disadvantage. The role of cytotoxic cells is to destroy mutated cells and tumours, much in the manner in which soldier drive back an invader. However, it is unfortunate that in the instance of transplants, the cytotoxic cells can often wrongly identify an organ that has been transplanted as a harmful invader and, hence, attack it as it would attack a virus or bacteria entering our body. In fact, cytotoxic cells are a major reason why many transplants are rejected by the body.
It is believed that T cells are very vital in fighting various autoimmune diseases, particularly the Acquired Immunodeficiency Syndrome (AIDS) and Human Immunodeficiency Virus (HIV). Several treatments that are being used currently for these two conditions entail triggering the production of these vital cells as well as their receptors to enable them to combat the harmful effects of HIV and AIDS. The "killer" T cells that occur naturally in our body do not possess the ability to fight HIV effectively. As a result, scientists have evolved various techniques to enhance these cells and turn the receptors of these cells more sensitive to this lethal virus.
As the study of how cells function is still somewhat new, science is still not able to understand much about the T cells and the manner in which they function. However, as scientists are continuously studying the effects of these helpful cells, it is thought that in due course reducing the transplant rejection due to the cytotoxic cells and providing enhanced treatment facilities for autoimmune disease will be possible.
T cells have their origin in haematopoietic stem cells that are made inside the bone marrow. A number of these multi-potent cells turn into progenitor cells and later depart from the bone marrow. Over time, these cells move on to the thymus through the bloodstream. Inside the thymus, these progenitor cells develop into T cells, which derive their name from their development in the thymus.
It is worth noting that in the thymus the T cells go through a selection process during which most of the maturing T cells (also known as thymocytes) do not survive. Thymocytes that come into contact with self-MHC molecules are the ones that receive positive signals and survive. On the other hand, thymocytes having receptors to self-antigen get negative signals and are subsequently done away with from the repertoire.
Once they survive, each T cell develops their individual T cell receptor (TCR), which is especially meant for a specific antigen (a substance that stimulates an immune reponse). T cells that succeed in surviving the selection process in the thymus (thymic selection) eventually mature and part from the thymus. Following this, they circulate throughout the peripheral lymphoid organs and each of them is prepared to come across a particular antigen and become activated. Having become activated, the T cell multiplies as well as differentiates into an effector T cell.
In this way, as we age, the thymus involutes and gradually starts producing fewer new T cells. In other words, the diversity of T cells in older people is much reduced. Hence, it is not surprising that they become more vulnerable to infections as they grow older.
There are three main varieties of lymphocytes and T cells are one among them. The other two types of lymphocytes are natural killer cells and B cells. The T cell lymphocytes are different from both B cells and natural killer cells, as the former contains a protein known as a T cell receptor which works to inhabit their cell membrane. The T cell receptors possess the capability to identify the various different types of antigens. Different from the B cells, the T cells never engage antibodies to combat germs.
On the other hand, T cell lymphocytes are also of various types, each having a particular function in our immune system. These different types of T cell lymphocytes include cytotoxic T cells, helper T cells, regulatory T cells, natural killer T cells and memory T cells. Each of these T cell types and their specific functions are described briefly below.
This type of T cell lymphocyte is engaged in directly destroying the cells that have turned carcinogenic or those that have been infected by any pathogen. Cytotoxic T cells enclose granules (which are minute sacs that contain digestive enzymes or other chemical materials) which are utilized by these cells to target an invading cell before starting a process that bursts the invading cell and destroy them. This process is known as apoptosis. Moreover, these T cells are also the major reason for transplant organ rejection. When an organ is transplanted in the body, cytotoxic T cells attack that organ as they attack any invading harmful cell, as it treats the transplanted organ as an infected tissue.
This type of T cell lymphocyte speeds up antibodies production by B cells. At the same time helper T cells trigger the cytotoxic T cells as well as white blood cells, which are called macrophages. HIV targets the CD4 +T cells and infects the helper T cells. As a result, HIV destroys the helper T cell by activating signals which lead to the death of T cell.
This type of T cell lymphocyte works to hold back the B cells as well as other T cells' responses to antigens (substances that trigger immune response). Such restraint is essential to ensure that the immune response does not prolong even when it is not needed. In fact, flaws in the functioning of the regulatory T cells often result in progress of autoimmune disease. When this disease occurs, the immune cells start attacking the healthy tissues of the body.
The name of this type of T cell lymphocyte is similar to another lymphocyte known as the natural killer cell. However, NKT cells are not natural killers as they do not kill harmful invading pathogens naturally. Nevertheless, the properties of NKT cells are similar to those of T cells as well as natural killer cells. Similar to all the other T cells, NKT cells possess T cell receptors. At the same time, the NKT cells share markers of many surface cells, which are similar to the properties of natural killer cells. NKT cells also possess the ability to detect infected or carcinogenic cells from among the normal cells in the body by itself. Having distinguished such infected cells, NKT cells attack the harmful cells not having any molecular marker that distinguishes them as cells of our body. Ivariant natural killer T (iNKT) cell is a type of NKT cell that works to shield the body from becoming obese. It achieves this by normalizing any kind of inflammation in adipose tissue.
This type of T cell lymphocyte aids the immune system to identify the antigens it has encountered earlier. They respond to such antigens for a prolonged period. Together, helper T cells and cytotoxic T cells can unite to form memory T cells. The spleen and lymph nodes store the memory T cells. In some cases, memory cells possess the ability to provide protection for a lifetime against particular antigen. As a result one is protected from enduring the same condition again.