BCRs Flashcards
where is BCR located?
B cells
What is BCR composed of?
- two identical heavy chains
- two identical light chains
- held together by disulphide bridges
what forms antigen-binding site?
variable regions of the heavy and light chains which allows the BCR to recognise specific antigens
what role does BCR play?
in adaptive immunity by allowing B cells to recognise a wide range of antigens and mount an effective immune response against them
what happens when an antigen binds to the BCR?
triggers a seris of intracellular signalling events that activate the B cell resulting in proliferation, differentiation into antibody-sescreting plasma cells and the generation of memory B cells
what does proliferation of B cells within the germinal centre lead to ?
- isotope switching of the antibodies from IGM and to IGG, IGA etc
- drive of affinity maturation
what are the two forms of BCR
1- Signalling receptor
2- Antibody
describe structure of membrane bound form of BCR
- hydrophobic residues at the carboxyl terminus of the heavy chain to anchor the BCR into plasma membrane
made of three sequentially arranged regions:
- extracellular hydrophillic region (spacer region about 26 AA)
- hydrophobic transmembrane segment (25 AA)
- short cytoplasmic tail inside of that membrane
whats the difference in structure between membrane bound BCR and soluble circulating BCR (antibodies)
the hydrophobic residues in the membrane bound are exchanged for more hydrophillic AA so they can be soluble and circulate throughout the blood
describe structure of membrane bound BCR further
- membrane-bound immunoglobulin non-covalently linked to a heterodimer composed of IGa and IGb
- this is in toa 1:1 stochiometry
what does the IGa and IGb heterodimer responsible for?
transducing the antigen signal once bound from the outside to the inside of the cell
describe IGa and IGb structure
- short n terminal Ig like domains at the top
- long cytoplasmic tail which contain immuno-receptor tyrosine based activation motifs (ITAMs)
what happens when antigen binds to BCR?
induces phosphorylation of ITAMs which results in the triggering of signalling cascades (CD79)
Whats another name for IGa and IGb heterodimer?
CD79
What phosphorylayes CD79?
LYN (Kinase)
describe the signalling cascade caused once CD79 is phosphorylated?
- CD79 phosphorylated by LYn cayses activation of other sites and kinases and signalling peptides such as BLINK, phospholipase C and BTK. forming a signalosome
- phospholipase C releases DAG which triggers RAS pathway and another pathway
- PPC also releases IP3 whihc can then activate Ca NFAT pathway
- leading to activation of B cell
describe all IgX
IgG- membrane bound and secreted
IgM - membrane bound and secreted as a pentameter
IgD- mainly acts as a receptor
IgA - membrane bound and secreted as a dimer
IgE - membrane bound and secreted
what is the antigen bindingsite made up of?
- 6 CDRs that protrude from the V domains
- CDRs are most variable regions of BCR responsible for specificity of antigen binding
- CDR1 and CDR2 short and located at N-terminal end of variable domamins
- CD3 is longest and most diverse
name all the moleculat interactions between CDRs and antigens?
- electrostatic
- hydrogen bonds
- van der waals
- hydrophobic interactions
- cation-pi interactions
what is the origin of cation-pi interactions?
non-covalent interaction between a cation and an electron cloud of a nearby aromatic group
what four things contribute to the specificity of CDRs?
1) AA sequence
2) Structural conformation - flexibility
3) Structural conformation - binding pockets
4) Affinity
how is AA sequence of CDRs generated?
through somatic recombination and somatic hypermutation - allowing for a wide range of possible amino acid sequences and a corresponding diversity of potential antigen binding sites
which CDR has the most number of contacts with the antigen?
CDR3
What three key things are important in flexibility of BCRs?
1) Adaptation to antigen shape
2) High specificity
3) High affinity
