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T-cells are a particular cell type that belongs to the category of White Blood Cells. This category is also called lymphocytes. These cells play an important role in the Immune System and more appropriately in the importance of adaptive cellular immunity.

Other cells that belong to this category includes: B Cell and natural killer cells.

T cells are identified by a specific cellular receptor called T cell receptors (TCR)

Several different types of T-Cells exist:

1.)  Helper T cells
2.)  Cytotoxic T cells
3.)  Gamma Delta T cells
4.)  Memory T cells
5.)  Natural killer T cells
6.)  Regulatory T cells

Helper T cells

–  Express a CD4 protein on their surface
–  Can assist other leukocytes such as B cells, cytotoxic T cells, and macrophages
–  Peptide antigens are presented to these cells
–  Cells become activated
–  When activated, Helper T cells divide rapidly causing a production and releases of cytokines.
–  These cytokines cause reactions in the immune response
–  Such reactions help in the regulation or continuation of the immune response
–  Several different subsets of Helper T cells exist

Cytotoxic T cells

–  Also referred to as CTLs and CD8+ T cells
–  Have a CD8 glycoprotein on their surface
–  These cells will destroy cells infected with a virus
–  These cells will destroy tumor cells
–  May have a role when a body has rejected an organ or tissue because of it being part of a transplant.

Gamma Delta T cells

–  Are a very small portion of T cells
–  A T cell receptor(TCR) is found in its surface.
–  This TCR is somewhat different than most T cells
–  This type of cell is often found in the gut mucosa
–  The overall function is not completely known

Memory T cells

–  Are found long after an infection has resolved
–  May have either CD4+ or CD8+ glycoproteins attached to their surface
–  Are apart of antigen-specific T cells
–  This cell type allows the immune system to have a “memory” of a previous infection
–  When body is re-exposed to the specific infection – this cell type becomes activated
–  Can expand quickly after an infection for protection
–  Two main types:  Central memory T cells and Effector memory T cells

Natural killer T cells

–  Bridge two systems of the immune system
–  Bridge Adaptive and Innate immune systems
–  These cells recognize antigen by a molecule called CD1d
–  Cells then become activated
–  They can do actions of both Helper T cells and Cytotoxic T cells
–  Can destroy some tumor cells
–  Can help against the Herpes Virus

Regulatory T cells

–  These were previously referred to as “suppressor T cells“.
–  Very important in immunological tolerance
–  Allow for the immune system to be controlled.
–  Specifically they stop or limit the T cell-mediated immunity by limiting the activity during this period
–  Several types of glycoproteins are on these cells such as CD4+, FoxP3 and others
–  Mutations of these genes can cause serious problems

Thymus

–  All T cells are produced from stem cells in the Bone Marrow
–  Thymus thus creates molecules or particles used by T cells
–  CD4- and CD8- are initiated here but require further progression become CD4+ and CD8+
–  These will be soon released from the thymus
–  98% do no make maturity while the other 2% progress to mature T cells
–  A large portion of T cells are produced at a younger age
–  The thymus will shrink as age increases

Positive Selection

–  A helpful aspect of being a T cell is the ability to interact with the MHC.
–  When a T cell has CD4+ and CD8+ it is allowed to interact with antigens.
–  Only these cells will be able to attract and identify.
–  Without this ability the complete function of the T cells is called into question.
–  Thus, those cells who can’t completely for their function are killed and degraded through Apoptosis
–  Positive selection also allows certain cells to become CD4+ cells and others CD8 cells.
–  This occurs when a cells with both CD4+ and CD8+ will actually downgrade one or the other.
–  Therefore they will become CD4+CD8- or CD4-CD8+

Negative Selection

–  This occurs when a proper cells, who appears functional, has a portion of its ability or functionality removed.