T CELL ACTIVATION.pptx
- 3. • Negative selection: The process by which T cells are screened so that those with a high affinity for binding to self antigens (and potentially causing autoimmunity) are destroyed. • Positive selection: The process by which T cells are screened so that only those capable of binding to MHC are kept alive.
- 4. THREE PHASES 1. ANTIGEN RECOGNITION PHASE 2. ACTIVATION AND DIFFERENTIATION PHASE 3. EFFECTOR PHASE T CELLACTIVATION AND DIFFERENTATION
- 5. T CELLACTIVATION • Activation of T cells occurs through the simultaneous engagement of the T-cell receptor and a co-stimulatory molecule (like CD4 or CD28) on the T cell by the major histocompatibility complex (MHCII) peptide and co- stimulatory molecules on the APC. • Both are required for production of an effective immune response; in the absence of co-stimulation, T cell receptor signalling alone results in anergy. – Anergy is a term in immunobiology that describes a lack of reaction by the body's defense mechanisms to foreign substances
- 6. FIRST SIGNAL • The first signal is provided by binding of the T cell receptor to its peptide presented on MHCII on an APC. • Peptides presented to CD8+ T cells by MHC class I molecules are 8–13 amino acids in length. • Peptides presented to CD4+ cells by MHC class II molecules are longer, usually 12–25 amino acids in length.
- 7. SECOND SIGNAL • The second signal comes from co-stimulation. in which surface receptors on the APC are induced by a relatively small number of stimuli, usually – products of pathogens, – sometimes breakdown products of cells, such as necrotic-bodies or heat shock proteins. The second signal licenses the T cell to respond to an antigen. Without it, the T cell becomes anergic, and it becomes more difficult for it to activate in future.
- 8. CD28