Adaptive Immunity Receptors (Week 4) Flashcards
Affinity vs. Avidity
affinity: high specificity of recognition
avidity: multiple recognition sites
Epitope
specific site of recognition on antigen
Antibody/BCR recognition
epitopes
- depend on folded conformation of the protein
- linear and discontinuous
Polypeptides in BCR/antibodies
- Four polypeptide chains
- Two identical heavy chains (50 kDa)
and two identical light chains (25 kDa)
Protein sequences in BCR/antibodies
- between antibodies/BCR have limited variation in constant regions and high variability in variable regions
- Both heavy and light chains have
a variable region (VH and VL)
What are the two identical arms on the BCR/antibodies called?
Fabs
fragment antigen binding
What is the stem of the BCR/antibodies called?
Fc fragment
- fragment crystallizable
BCR diversity
- We have a vast array of BCRs
that can bind to a large number of potential pathogens - Changing one amino group on
an epitope can influence antibody binding - many different theories on how it arose
Germline Theory
- one gene for every BCR
- impractical
- How can a genome that is smaller than this hold all those genes?
Somatic Mutation Theory
- Suggests that a limited number of antibody genes undergo mutations to generate diversity
- mutations to small number of BCR genes only in B cells to alter and increase BCR diversity.
- no effect to germline genes
DNA recombination
- accepted model
- Antibody producing cells are not only missing a gene but also have smaller genes coding for antibody peptides
- many different processes
Somatic recombination
multiple germline genes encode for variable regions, genes are cut and spliced to create BCR diversity
What DNA segments are required for the light chain locus?
- Variable (V)
- Joining (J)
- Constant (C)
- Variable region = V and J
What DNA segments are required for the heavy chain locus?
- Variable (V)
- Joining (J)
- Diversity (D)
- Constant (C)
- Variable region = V, D and J
What segments are the most variable in length?
Diversity (D) segments
Complementarity Determining
Region (CDR)
where the BCR interacts with antigen
Where is light chain diversity generated?
between V and J segments
Where is heavy chain diversity generated?
mostly within and around D segment
What chromosomes are heavy chain genes?
14
What chromosomes are light chain genes?
either 2 or 22
Recombination Signal Sequences (RSSs)
- next to every gene segment and are either 7 or 9 nucleotides
- guide recombination
- Every pair consists of one 7 nucleotide and one 9 nucleotide
RSS separated by a spacer of either 23 or 12 nucleotides - “heptamer-spacer-nonamer”
12/23 rule
- recombination can only occur between segments that have
spacers of different lengths - segment with a 12-bp spacer must recombine with a segment containing a 23-bp spacer
- Ensures correct order of VDJ segments with no repeats
What are the recombination activating genes?
RAG1 and RAG2
- are unique to B and T cells
How do RAG proteins initiate V(D)J recombination?
- complex recognizes and binds
two distinct RSSs creating a hairpin - complex cleaves DNA to recombine
the coding joint and exclude unnecessary regions in the signal
joint
Recombination enzymes and additional diversity
- RAG complex opens hairpins that it
generates after cleaving heptamers - Terminal deoxynucleotidyl
transferase (TdT) adds random nucleotides to the open hairpins - Once the two segments are able to pair, Remaining unpaired nucleotides are cleaved, DNA polymerase and ligase fill in remaining sections
