Cytotoxic lymphocytes (CTLs) are a specific type of white blood cell that is part of the human body's immune system. These lymphocytes are commonly referred to as killer T-cells for their ability to seek out and destroy diseased cells within the body. CTLs belong to a specific group of double positive T-cells known as CD8+ and CD4+ cells, which develop in the thymus gland.
All cytotoxic lymphocytes begin life as stem cells in the bone marrow. From there, they enter into the thymus gland, where they undergo a transformation to become specialized killers. The first type of cytotoxic lymphocytes, CD8+ cells, are programmed to kill cells in the body that have become diseased by viruses, cancer, parasites or fungus. The second type, CD4+ cells, are helper cells that identify diseased cells and put out a distress call to macrophages, which come in and destroy the diseased cells.
CD8+ cytotoxic lymphocytes find infected cells by identifying foreign proteins being emitted by the contaminated cell. After the cell is located, the lymphocyte attaches itself to the outer wall using a protein called perforin. The perforin drills tiny pores into the cells plasma membrane.
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After the pores have been created, CTLs inject another type of proteins called granzymes into the diseased cell. Granzymes enter the mitochondria of the infected cells and pull it apart. The final stage of cell destruction occurs when the granzymes program the host cell to self-destruct in a process called apoptosis. By destroying the host cell completely, it alleviates the possibility of the virus that has infected it replicating and creating new viruses, thus spreading the infection.
It was once thought that cytotoxic lymphocytes targeted only cells that had been infected with viruses and possibly cancerous cells. It is now known that CTLs also target cells that have been infected with fungus, parasites and bacteria. CTLs are capable of killing nearly any type of cell within the body, and each individual CTL is programmed to identify a specific antigen that is being emitted by a foreign substance within a cell.
In cases of autoimmune disorders, CTLs have gone awry. Multiple sclerosis, type 1 diabetes and Hashimoto's disease, for example, demonstrate how cytotoxic lymphocytes can misidentify normal cell transmissions as disease markers. CTLs attack these healthy cells, causing inflammation at the attack sites, leading to tissue damage or a decreased number of cells that are essential to specific bodily functions being performed properly. Cytotoxic lymphocytes also are contributors to the body's rejection of organs that have been replaced in instances such as heart transplant or liver transplant operations.