[W7] Immunogenetics 2 Mechanisms for generating Ab and TCR diversity Flashcards
(25 cards)
What gene segments are used in antibody genes?
V (Variable), D (Diversity – heavy only), J (Joining), and C (Constant).
What happens to DNA during B cell development?
DNA is rearranged, and some segments are permanently lost.
Which enzyme initiates DNA cleavage at RSS sites?
RAG1 and RAG2.
What is the structure of an RSS?
A nonamer + heptamer, separated by 12 or 23 base pairs.
What does the 12/23 rule ensure?
Correct pairing of gene segments during recombination.
How are many Vκ segments arranged in humans?
Inverted, requiring complex looping for recombination.
What is P-nucleotide addition?
Addition of palindromic sequences from hairpin loop cleavage.
What is N-nucleotide addition and what enzyme mediates it?
Random nucleotide addition by TdT at junctions.
What is a possible outcome of imprecise joining?
Non-productive rearrangements (e.g., out-of-frame sequences).
What does AID do to DNA?
Converts cytosine to uracil, triggering mismatch repair or base excision.
What are the two main processes requiring AID?
Class switching and somatic hypermutation.
What happens in AID knockout mice?
No isotype switching or affinity maturation occurs.
What is isotype switching?
Changing the antibody class (e.g., IgM → IgG) without changing antigen specificity.
What sequences mediate isotype switching?
Switch (S) regions upstream of each constant region.
How is DNA rearranged during switching?
Loops out intervening DNA to join two S regions (e.g., Sμ to Sγ).
What determines whether IgM or IgD is made initially?
Alternative polyadenylation of the primary transcript.
Can the same B cell produce both secreted and membrane Ig?
Yes – through alternative splicing.
What activates transcription of switch regions?
Cytokine stimulation (e.g., IL-4, IFN-γ) and transcription factors.
What happens after AID deaminates cytosines?
DNA repair systems induce double-strand breaks, enabling recombination.
Where does somatic hypermutation occur?
In the germinal centres of lymph nodes.
What region of the antibody is most affected?
CDR1 and CDR2 of the V region.
What drives somatic hypermutation?
AID, induced after antigen stimulation.
What did studies of anti-hapten antibodies show?
Mutations accumulated in the same germline V genes, improving affinity.
How does the mutation rate change with repeated exposure?
Increases with boosters and secondary responses.
