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T-cells

T-cells are coordinators of immune regulation and enhance the body's immune responses through secretion of specialized factors which activate white blood cells that fight off infection. These cells assist in detecting immune deficiencies and destroy invaders that try to attack our body.

T-cells are produced in the bone marrow and mature in the thymus. When these cells get matured, they travel in the blood to other lymphoid tissues, such as the spleen, tonsils, and lymph nodes. They play a vital role in protecting our body against diseases by eliminating cancerous cells and those infected with virus and bacteria.

T-cells Function

T-cells are produced in the bone marrow and move to the thymus gland to mature. There are two important steps which decide the maturity of T-cells. In the first step T-cells should recognize major histocompatibility complex (MHC) because they play an important role in maintaining the immune system and autoimmunity. MHC is found on the surface of all cells and signals an alarm if a virus is reproducing in the cell.

The second test is that, any T-cells that recognize peptides that are normally in the body will be killed. After the recognition of peptide in the body, it goes into effector mode and changes from a naive T-cell into an effector T-cell.

There are very few T-cells that leave the thymus gland and are properly functional. There are different types of T-cells and each of these types has important functions. Some of these are described below.   

T-cells types

  • Suppressor T-cells

CD8 +T-cells are called suppressor cells or regulatory cells because they are involved in killing viral infected cells, tumor cells and parasites. These cells are important because they down regulate immune responses.

  • Natural killer cells

Natural killer cells (NK) are similar to CD8+T-cells and work as effector cells by killing tumors like lymphomas, melanomas and viral infected cells such as herpes and cytomegalovirus. Lymphoid organs are generally targeted by these cells. CD4 +T-cells which are activated through the secretion of natural killer cells kill viral infected tumors more efficiently.

  • T helper cell

The immune system is made of number of cell types which can kill parasitic or tumor or viral infected cells. These cells are dependent upon the, 'T,' helper subset for activation signals in the form of secretions >which are referred to as, Lymphokines, Cytokines or Interleukins. T helper subsets assist in detecting immune deficiencies.

  • Cytotoxic T-cells

Cytotoxic T-cells are related to a sub group of T lymphocytes that are capable of destroying tumor cells. These cells are alerted by helper T-cells or cell membrane proteins that body cells are infected with a pathogen and then check the body cells for pathogen proteins on their cell membranes. Cytotoxic T-cells are affected by variety of mechanisms and they can destroy antigens directly by releasing cytotoxic substances. These cells are very effective against tuberculosis and cancer cells, as they attract macrophages and intensify phagocytosis.

  • Memory T-cells

Memory T-cells provide excellent immune action against various infections, pathogens and foreign bodies. They are capable of recollecting memory against past infections and quickly expand to large number of effector T-cells upon reoccurrence of that infection. Memory T-cells can be classified according to central memory T-cells and effector memory T-cells.

T-cells in the immune system

The human immune system is a protection mechanism which defends the body against millions of bacteria, microbes, viruses, toxins and parasites that try to invade the human body. Our immune system can detect viruses, parasitic worms and differentiate them from healthy cells and tissues, required for the normal functioning of the body.

T-cells play a crucial in maintaining a healthy immune system as they help the immune cells to recognize specific invaders (antigens) remember them and launch an attack against them if they are encountered again. The white blood cells known as lymphocytes, that include T-cells destroy antigens directly. The lymphocytes act as the defending army of the human immune system providing a shield against several diseases and viruses.

A healthy immune system with adequate T-cell count helps our body to withstand damage from anti- biotic resistant bacteria and prevents the reoccurrence of pathogens hiding in the body like herpes that causes cold sores and mycobacterium that causes tuberculosis

T-cells help maintain fine tuned immune reactions to any antigen that trigger an immune response by managing cell mediated system and helping the B cells to manage humoral system.  They play a vital role in destroying infected or cancerous cells and coordinate all Acquired Immune Responses.

Natural killer (NK) cells are a type of T-cells which activate and regulate immune elements and independently handle the destruction of tumor cell by producing Cytokines.

T-cell Lymphoma

Lymphoma is a cancer of the white blood cells, namely lymphocytes that constitute the lymphatic system. The two main types of lymphoma are Hodgkin lymphoma and non-Hodgkin lymphoma.

T-cell lymphoma is a rare disease in which T lymphocyte cells become cancerous. These lymphomas account for between 10 percent and 15 percent of all cases of non-Hodgkin lymphoma in the United States (approximately 5,000 to 6,000 cases) a year. However, some forms of T-cell lymphoma are more common in Asia, Africa and the Caribbean.

There are many different types of T-cell lymphoma, most of which are extremely rare, occurring in only a few patients per year throughout the world. Like the B-cell lymphomas, T-cell lymphomas are classified into two broad categories - aggressive (fast-growing) or indolent (slow-growing).

Peripheral T-cell lymphoma (PTCL) comprises a group of rare and aggressive non-Hodgkin lymphomas that develop from T-cells in different stages of maturity.

The World Health Organization (WHO) has divided the various types of PTCL into two main categories

  • precursor T/NK neoplasms
  • peripheral T/NK neoplasms

Peripheral T-cell lymphoma generally affects people over age 60 and is diagnosed in slightly more men than women.

T-cell Leukemia

T-cell leukemia/lymphoma (ATL) is a rare cancer of the immune system's own T-cells.  Human T cell leukemia/lymphotropic virus type 1 (HTLV-1) is believed to be the cause of it.

ATL is a highly aggressive non-Hodgkin's lymphoma with no characteristic histologic appearance except for a diffuse pattern and a mature T-cell phenotype.

Some of the types of T-cell leukemia include:

  • Large granular lymphocytic leukemia
  • Adult T-cell leukemia/lymphoma
  • T-cell prolymphocytic leukemia

Often, it is difficult to distinguish T-cell leukemia from T-cell lymphoma, and they are often grouped together.

T-cell receptor

The T cell receptor or TCR is a molecule found on the surface of T lymphocytes (or T cells) that is responsible for recognizing antigens bound to major histocompatibility complex (MHC) molecules. 

It is a heterodimer consisting of an alpha and beta chain in 95% of T cells, whereas 5% of T cells have TCRs consisting of gamma and delta chains.

Engagement of the TCR with antigen and MHC results in activation of its T lymphocyte through a series of biochemical events mediated by associated enzymes, co-receptors, specialized accessory molecules and activated or released transcription factors.

T-cells count

T-cells count is a type of blood test which measures the number of T-cells in blood. This test determines whether the T-cells count is low or high. This test also determines the condition of person’s immune system.

This test is very beneficial for the doctor to administer patients suffering from HIV, as it gives them an idea about the damage done to the immune system by this virus. The normal range of CD4 T-cells is usually between 400 and 1600 and people suffering from HIV infection have CD4 T-cells count below 200.

The total count of these cells in the blood also reflects total lymphocyte count in the body, which can be temporarily decreased due to infections. 

T-cells higher than normal levels are due to multiple myeloma, acute lymphocytic leukemia and infectious mononucleosis.

Lower T-cells count may occur due to several reasons:

  • AIDS
  • Acute viral infection
  • Aging
  • Cancer
  • Leukemia
  • Radiation therapy

Low T-cell count risk

Lower T-cell count results in weak or defective immune system. This leads to increased susceptibility to infection, allergies, autoimmune diseases and sometimes even cancer.

There are some immune deficiency diseases which are acquired. Human immunodeficiency virus (HIV) is one of the acquired immune deficiency diseases. This virus that causes AIDS (Acquired Immunodeficiency Syndrome) destroys the immune system of our body. People with AIDS have very low T-cell count and thus lose the ability to fight against infections. 

Intake of immunosuppressant medications is also one of the reasons for having low T-cell count which generally leads to immunodeficiency diseases.Severe combined immunodeficiency (SCID) is a type of immune deficiency which is caused by the lack of B and T lymphocytes. Low count of T lymphocytes makes it impossible for the immune system to fight against any infections.