White Blood Cells 2

T cells must recognize nonself Ag complexed to self MHC.

Most mature T cells express either CD4 or CD8 and have an Ag-binding, Ig-like surface receptor called the T-cell receptor (TCR).

As for B cells, the number of T-cell specificities is almost limitless.

For T cells to be activated, the TCR must engage with Ag-MHC.

Most T lymphocytes express one of two antigens — CD4 and CD8. CD4 is expressed by helper and inflammatory T cells and is a co-receptor for MHC class II molecules. CD8 is expressed by cytotoxic T cells and is a co-receptor for MHC class I molecules. An increased CD4/CD8 ratio commonly indicates heightened immune function, whereas a decreased ratio is indicative of prevalent disease.

_____ T-cell (type) are usually CD4 but may be CD8. They differentiate from TH0 cells into one of the following:

TH1 cells: In general, TH1 cells promote cell-mediated immunity via cytotoxic T cells and macrophages.
TH2 cells: TH2 cells promote Ab production by B cells (humoral immunity).
TH17 cells: TH17 cells promote tissue inflammation.

yes,the distinction between the different TH cells is clinically relevant. For example, a TH1 response dominates in tuberculoid leprosy, and a TH2 response dominates in lepromatous leprosy. A TH1 response is characteristic of certain autoimmune disorders (eg, type 1 diabetes, multiple sclerosis), and a TH2 response promotes IgE production and development of allergic disorders, as well as helps B cells produce autoantibodies in some autoimmune disorders (eg, Graves' disease, myasthenia gravis). TH17 cells, via their role in inflammation, may also contribute to autoimmune disorders.

Regulatory T cells mediate suppression of immune responses. The process involves functional subsets of CD4 T cells that either secrete cytokines with immunosuppressive properties or suppress the immune response by poorly defined mechanisms that require cell-to-cell contact. Some regulatory T cells express the CD8 T-cell phenotype.

Cytotoxic T (TC) cells are usually CD8 but may be CD4; they are vital for eliminating intracellular pathogens, especially viruses. TC cells play a role in organ transplant rejection.

Fully activated TC cells, like NK cells can kill an infected target cell by inducing apoptosis