All T-cells have CD3 receptor complexes and various other CD antigens. CD8 antigens are found on TC-cells and TS-cells. CD8 containing cells respond to antigens displayed on Class I MHC proteins. CD4 antigens are found on TH-cells, and respond to antigens displayed on MHC II proteins. CD4 and CD8 markers are bound to the CD3 complex on the T-cell surface. Activation does not usually occur until the T-cell encounters and recognizes (binds) a second antigen of the same type. The “double-signature” is a double-check before destruction.
There are two types of cells that carry CD8 antigens – cytotoxic T-cells and suppressor T-cells.
Cytotoxic T-cells
Much like B-cells, once activated, cytotoxic T-cell (TC-cells) begin dividing to produce more TC-cells, and TC-memory cells. TC-cells destroy cells containing the antigen they are specific for; whereas, CD8 containing memory cells immediately differentiate to TC-cells in response to future exposure to the same antigen. TC-memory cells result in a swift cytotoxic response.
Suppressor T-cells
Suppressor T-cells (TS-cells) produce inhibitory cytokines for T-cells and B-cells. TS-cells are slow to respond and act only after the initial immune response.
CD4 containing cells divide to produce helper T-cells (TH-cells) and memory TH-cells. CD4 markers are contained on the surface of TH-cells.
CD4 stimulation results in:
- cytokine production to promote T-cell differentiation and division (TH-cell and TH-memory cell production),
- acceleration of TC-cell maturity,
- macrophages attration,
- NK cell activity stimulation ,
- B-cell activation.
TC-cells contain CD8 receptors. CD8 receptors recognize foreign or abnormal amino acids displayed on MHC-I containing cells of infected or cancerous cells. Activated TC-cells then kill the targeted cells to prevent replication.
The weapons arsenal of activated TC-cells includes release of a chemical agent known as perforin, secretion of a poisonous chemical known as lymphotoxin, and activation of programmed cell death or suicide (apoptosis).
TH-cells are also referred to as T4-cells because they carry CD4 on their cell surfaces. T4-cells recognize foreign antigen fragments displayed on MHC-II containing cells and secrete cytokines to improve immune response. T4-cells are essential for activation of B-cells, additional T-cells, natural killer (NK)-cells, and macrophages in response to bacterial, viral, parasitic or fungal invasion.
T4-cells also bind to B-cells and promote their differentiation into plasma cells and memory cells. There are two subpopulations of TH cells – TH1 and TH2.
CD4 is the receptor for the human immunodeficiency virus (HIV): T4-cells become infected with HIV, over time resulting in acquired-immune deficiency syndrome (AIDS) due to loss of T4-cells and the immune response capabilities provided by them.
Source:
Blood-borne Pathogens Comprehensive
