What Is a T Lymphocyte? The Key to Immune System Treatments

When the body’s innate immune mechanismsare insufficient against harmful agents, the acquired (adaptive) immune system comes into action. Acquired immunity is the responsibility of lymphocytes . There are 3 main types: 

• B lymphocytes: they participate in the humoral (fluid-based) response of acquired immunity through the antibodies they produce.
• T lymphocytes: they participate in the cellular response of acquired immunity by directly killing cells and regulating other immune cells.
• Natural killer lymphocytes (NK): they kill infected and tumor cells marked by antibodies and participate in innate immunity.

Among these defense cells, T cells in particular have a special role in cancer processes and new-generation immunotherapies.

What is a T lymphocyte?

T cells, involved in the cellular defense of acquired immunity, differentiate, like all blood cells, from bone marrow stem cells .

In the first differentiation stage, precursor T cells form, partially losing stem cell characteristics. If precursor T cells do not reach the thymus gland, they become NK cells. Those reaching the thymus differentiate into two types: cytotoxic T cells (CD8+) and helper T cells (CD4+).

What are the types of T lymphocytes? How do T cells recognize antigens?

Foreign antigens can be recognized only if presented on antigen-presenting cells’ MHC proteins by T-cell receptors . Cytotoxic (killer) T cells recognize antigens presented by MHC class-1 proteins on normal body cells. Helper T cells recognize antigens presented by MHC class-2 proteins. When a pathogen is broken down inside a cell, the resulting antigen combines with an intracellular MHC protein and is incorporated as a complex into the cell membrane.

MHC class-1 proteins are present on all nucleated cells, whereas MHC class-2 is found only on antigen-presenting cells. Cytotoxic T cells recognizing an antigen secrete perforin, which makes holes in the infected cell’s membrane. Another way they cause its death is by activating its apoptosis mechanism. Helper T cells secrete cytokines to stimulate other immune system cells or bind B cells and cause them to become plasma cells producing antibodies specific to that antigen. Together, these mechanisms form cellular immunity, the final line of defense after a pathogen enters the cell.

The most advanced example of using T cells in cancer: the living drug CAR-T cellular therapy

"Chimeric Antigen Receptors” is abbreviated CAR-T; this cell therapy is fully personalized. The patient’s T cells are trained with genetic techniques to attack cancer cells. First, T cells are isolated from a blood sample. In the laboratory, genes are transferred into them to recognize antigens on cancer cell surfaces. These genetically modified T cells are then returned to the patient. Producing this personalized drug takes about 3 weeks.

Because living cells are administered to the body, this treatment is also called a living drug .

In 2010, after a diagnosis of acute lymphoblastic leukemia at age 5, Emily Whitehead’s story introduced CAR-T therapies to the entire oncology world.

Article image: T cells attacking a cancer cell are depicted.