Antibodies and B Cells Flashcards
What is the total number of antibody specificities available to a human individual (also known as the antibody repertoire)
10^11 at least
In the genomic organisation of the light chain in the antigen binding site, what are the 3 gene segments?
Variable (V) = ~40
Joining (J) = 5
Constant (C) = 1
In light chain DNA remodelling, what two gene segments are joined by the excision of a whole series of V and J genes in order to create a new VJ combination
Joining of Random V and Random J gene
What happens after the somatic recombination of the light chain DNA
The somatically recombined DNA is transcribed into pre mRNA and then the introns are spliced out, combining the V, J and C regions. The message is translated into the light chain protein
What additional gene segment is found in heavy chains?
Heavy chains have up to 27 addition D (diversity) region genes.
What happens during the somatic recombination of heavy chain genes?
Joining of D-J regions followed by V-DJ joining
Name the locations of the 2 immunoglobulin chains loci?
Light Chains: k genes are on chromosome 2 and h genes are on chromosome 22
Heavy Chains: On chromosome 14
What 2 recombinase enzymes direct the breakage of the genes during somatic recombination?
Rag1 and Rag2
What enzyme repairs the rejoined segments
DNA Ligase
What are recombination signal sequences??
Conserved DNA heptamer/nonamer sequence flank the coding regions adjacent to the gene segments. Rag1 and Rag2 bind to these sites randomly and cleave these sites. Ligase joined to ends together
What binds the newly cleaved DNA ends together
Ku70:Ku80
What does DNA-PK:Artemis do?
Opens hairpins of the two cleaved ends
What is Omenn Syndrome?
Caused by missense mutation in RAG1. Antibody gene rearrangement cannot occur. Gatal without bone marrow transplantation. Characterised by skin redness, peeling skin, hair loss, chronic diarrhea, enlarged lymph nodes, liver, spleen, lower numbers of lymphocytes and IgG etc..
What is Class Switching?
B cell producing IgM can start producing another Ig with a the same specificity for antigens
What determines the class of antibody?
The constant (C) gene segment. One C exon is replaced by another and deletion occurs in the intervening sequence
What is the effect of allelic exclusion?
Mechanism in which only one (either the maternal antibody genes or paternal) is expressed with the other is switched off
Why does antibody affinity for antigens increase during the course of an immune response after gene rearrangements have taken place???
Hypermutation. Activated B cells make a deaminase (AID) that converts cytosine to uracil in Ig DNA variable regions. This occurs when cells try to fix the error. B cells then start to produce antibodies with higher affinity for antigen then proliferate
What removes Uracil after hypermutation?
Enzymes UNG and APE1. This forms a single-stranded nick in DNA. This can then be replaced with A, T, C or G
Where does B cell proliferation and somatic hypermutation occur?
In the germinal centre of the lymphoid tissue
What are centroblasts?
Rapidly proliferating B cells and occupy the dark zone of the germinal centre
How many chains are found in Ig’s?
2 heavy chains and 2 light chains (4)
What holds the light and heavy chains together and also the 2 heavy chains together?
Inter-molecular disulphide bonds
Where is the Fc region?
Tail region of the antibody
Where is the Fab Region?
The region containing the antigen-binding site
How many antigens can the antibody bind to at once?
2
