Recognition of Antigen by the Adaptive Immune System NOT FINISHED Flashcards
Define antigens
Molecules that induce an immune response through the activation of antigen specific lymphocytes
What are the 3 groups of molecules that work together to recognise antigen for the adaptive immune system?
- Major histocompatability complex (MHC)
- B cell receptors
- T cell receptors
What is the first response of a B/T cell when it encounters an antigen, that its receptor is specific for? Why?
They expand in number.
To increase number of cells able to respond to pathogen
Lymphocytes persist post-infection –> memory
There are a huge range of antigens, therefore B and T cells have to be very diverse. How do they achieve this?
- gene rearrangement of the different parts encoding the receptor (combinatorial diversity)
- B cell receptors achieve diversity and further specificity for antigen through a process of rapid mutation called hypermutation
What is the different between peptides that B cells and T cells are able to recognise? How do they overcome this?
- T cells can only recognise short peptide sequences from antigens. The pathogen must be broken down and processed into smaller peptides within an antigen presenting cell and then present on their surface by a presenting molecule such as MHC
- B cells can recognise while antigen through their B cell receptor which binds to surface molecules or patterns. Soluble antigen receptors (antibodies/immunoglobulin) are then released from terminally differentiated B cells. Antibodies are then very effective at presenting antigen to T cells
What is the simple difference between major histocompatabiity complex class 1 and 2?
Class I recognises peptides derived from endogenous proteins
Class II recognises peptides processed from exogenous proteins
Where are Class I MHC molecules found?
on all nucleated cells, except neurones
What two things do MHC class I molecules allow the recognition of?
infected cells i.e. by viruses
abnormal changes occurring in a cell i.e. cancer
What is proteolytic activity?
How does this relate to MHC activity?
The continuous process by which the cells of the body break down a proportion of the proteins they are producing. Most amino acids are recycled but some are transported through ER to surface and loaded onto the peptide-binding groove of a class 1 MHC molecule. --> viral proteins are expressed on the cells surface
How is the MHC Class I: peptide complex recognised and what is the result of this?
Recognised by cytotoxic T cell specific for that viral peptide
Cytotoxic T-cells express CD8 (coreceptor for TCR).
CD8 facilitates interaction between T cell and MHC Class I on infected cell.
Triggers cytotoxic T cell to kill the virally infected cell by inducing apoptosis.
The cell is then consumed by macrophages
What are the rolls of the following in MHC Class I mechanism of working?
- TAP protein
- calnexin and calreticulin
- Tapasin
- TAP protein controls the movement of viral proteins into the ER
- Calnexin and calreticulin assists in the folding of MHC. These are needed because the protein is relatively unstable before it has bound peptide.
- A pool of MHC is retained in the ER by the protein Tapasin, so that a rapid increase in surface peptide/MHC complexes can occur following peptide binding
Which cells express MHC II?
antigen presenting cells
e.g. dendritic cells, monocytes/macrophages and B cells
How is MHC II protected from picking up antigen before it reaches the ER?
- complexed with a protein called the invariant chain
- degraded in the ER and epitope is inserted
Which cells do MHC II present antigen to?
CD4 T cells
How does CD4 interact with MHC II. What events follow this interaction?
T helper cells express CD4 as well as TCR’s. When CD4 interacts with MHC II its TCR can meet and become specific for the antigen being presented.
Depending on cytokine environment, T cell polarises to become a memory cell of effete cell (Th1, Th2 and T regulatory and Th17 cells
