TCRs & Antigen Presentation Flashcards
T-cell receptor
The TCR binds linear arrays of amino acids
It does not recognise native antigen
There are two types of TCR
- aB
- yd
TCR signalling
The TCR itself does not carry out any signalling
Instead, it uses CD3; a molecule of 6 polypeptides, each with one ITAM
CD4 and CD8 also play a role in T-cell signalling
CD8 T-cells
Recognise MHC class I molecules
CD4 T-cells
Recognise MHC class II molecules
T-cell activation
T-cell activation occurs when the TCR interacts with the MHC/antigen complex
CD4/ CD8 then join this complex
CD3 then initiates intracellular signalling
Antigen processing
The antigen fragments are then loaded into an MHC complex
MHC class I
Found on all nucleated cells and platelets
Involved in antigen recognition of virally infected cells or altered self-material
Present to CD8 T-cells
MHC Class II
Found on professional antigen presenting cells
Present to CD4 T-cells
Dendritic cells
Dendritic cells are a key link between innate and adaptive immunity
When they encounter antigen, they translocate to the lymph nodes where they activate T-cells
MHC Class I - loading mechanism
Carry out the endogenous pathway
Antigens come from within the cell
They are processed by the proteosome and then imported into the ER by TAP proteins
The antigen fragments are then loaded into the MHC with the help of calnexin and ERp57
MHC Class II - loading mechanism
Carry out the exogenous pathway
Antigens come from outside the cell
MHC is produced in the ER and packed into vesicles; invariant chain (Li) blocks the binding site
The vesicles fuse with endocytosed vesicles containing antigen
Li is partially degraded, leaving behind CLIP which is then displaced by the peptide with the help of DM
Non-classical T-cell activation
y/d T-cells make up 5% of all T-cells in the body
They are found in epithelial-rich tissues
They recognise antigen presented by CD1
Function unknown
Superantigens
Proteins secreted by extremely virulent bacteria and viruses
Able to bypass normal antigen recognition and bind non-variable regions of the TCR, causing activation
High effective even at low doses
