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.
