Antigen recognition: T cells and MHC Flashcards
(49 cards)
What are the equivalent of antibodies on T cells?
T cell receptors
How are T cells different from B cells?
T cells carry different receptors
T cells are encoded by different loci
B cells evolved after T cells
What evidence is there that B cells are more evolved that T cells?
T cells are found in simpler animals
How are the T cell receptors structurally different from B cell receptors?
TCR is made up of alpha and beta chains
BCR are made of a dimer of dimers
What is referred to when we say T cell receptors?
Receptors that specifically recognise antibodies
NOT the receptors for different cytokines and antibodies
How are BCR and TCR fundamentally different?
TCR are solely transmembrane
BCR can be secreted
TCRs are heterodimers, BCRs are dimers of dimers
Which mechanism of diversity exists in BCR but not TCR generation?
Somatic hypermutation
Which mechanisms of cell diversity are similar to B cells?
Multiple germline genes
V-J and V-D-J recombination
Recombinational inaccuracies
N-nucleotide addition
Chain combination
Describe the evolution of TCRs and BCRs
T and B cells are homologous
This is made obvious by their similarities
They are derived from one common ancestral gene that later evolved separately
How many times has the locus producing the T cell receptors evolved?
At least 3 times
What are the two types of TCRs found on T cells?
TCR ab
TCR gd
How are the two types of TCR different?
TCR ab is very common, occupying 50% of circulating lymphocytes
TCR gd is expressed by a distinct population of T cells with less variability than ab T cells, since there are less genes coding for them
What is the difference between the ways BCR and TCR recognise antigens?
BCR complementarity depends on the shape of the antigen
TCR complementarity does not depend on 3D shape, but rather recognises sequences of the antigen
Why is the way TCRs recognise antigens more efficient?
The shape of an antigen changes very readily as it evolves, so a recognition system based on this is not very efficient
T cells recognises smaller pieces of the pathogen which are difficult to change
What are MHCs?
Major Histocompatibility Complexes
Molecules which act to bind to the highly unstable peptides which T cell receptors recognise
What is another name for MHCs?
HLA
MHCs in humans
What is the shape of MHCs?
Single alpha chain composes the whole structure
Beta-2 microglobulin holds the structure together
Contain grooves where the antigen fits into
What is the shape of the groove antigens fit onto the MHC through?
Two a-helices
A b-pleated sheet floor
What happens when the peptides bind to the MHC?
Peptide becomes deeply embedded
Becomes an intrinsic part of the structure
Without it, the MHC is unstable
What do T cell receptors recognise?
T cells can only detect the amino acids exposed through the MHC
This hospot (antigen + MHC) is detected as non-self
So the TCR recognises the composite structure formed by the mixture of self (MHC) and non-self (viral/bacterial peptide)
What type of binding occurs between the MHC hotspot and the TCR?
Covalent bond
Must be tightly controlled in order to retain the characteristic weak binding that remains a feature of T cells
This allows T cells to leave once bound to its target
How specific must the MHC binding groove be?
The specificity must be tightly controlled
If it is too specific, no peptide would bind to it
If it is too broad, no peptide would bind strongly enough
One MHC binds to one peptide
TRUE or FALSE
FALSE
One MHC binds to a lot of different peptides
MHC have pockets which only take amino acids which fit in specific positions
Investigations have shown the peptides that fit to a specific MHC have some similarities between them
What type of interaction occurs between the MHC and the peptide?
Non-covalent
