Antibodies and B Cells

Question 1

What is the total number of antibody specificities available to a human individual (also known as the antibody repertoire)

Answer 1

10^11 at least

Question 2

In the genomic organisation of the light chain in the antigen binding site, what are the 3 gene segments?

Answer 2

Variable (V) = ~40
Joining (J) = 5
Constant (C) = 1

Question 3

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

Answer 3

Joining of Random V and Random J gene

Question 4

What happens after the somatic recombination of the light chain DNA

Answer 4

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

Question 5

What additional gene segment is found in heavy chains?

Answer 5

Heavy chains have up to 27 addition D (diversity) region genes.

Question 6

What happens during the somatic recombination of heavy chain genes?

Answer 6

Joining of D-J regions followed by V-DJ joining

Question 7

Name the locations of the 2 immunoglobulin chains loci?

Answer 7

Light Chains: k genes are on chromosome 2 and h genes are on chromosome 22
Heavy Chains: On chromosome 14

Question 8

What 2 recombinase enzymes direct the breakage of the genes during somatic recombination?

Answer 8

Rag1 and Rag2

Question 9

What enzyme repairs the rejoined segments

Answer 9

DNA Ligase

Question 10

What are recombination signal sequences??

Answer 10

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

Question 11

What binds the newly cleaved DNA ends together

Answer 11

Ku70:Ku80

Question 12

What does DNA-PK:Artemis do?

Answer 12

Opens hairpins of the two cleaved ends

Question 13

What is Omenn Syndrome?

Answer 13

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..

Question 14

What is Class Switching?

Answer 14

B cell producing IgM can start producing another Ig with a the same specificity for antigens

Question 15

What determines the class of antibody?

Answer 15

The constant (C) gene segment. One C exon is replaced by another and deletion occurs in the intervening sequence

Question 16

What is the effect of allelic exclusion?

Answer 16

Mechanism in which only one (either the maternal antibody genes or paternal) is expressed with the other is switched off

Question 17

Why does antibody affinity for antigens increase during the course of an immune response after gene rearrangements have taken place???

Answer 17

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

Question 18

What removes Uracil after hypermutation?

Answer 18

Enzymes UNG and APE1. This forms a single-stranded nick in DNA. This can then be replaced with A, T, C or G

Question 19

Where does B cell proliferation and somatic hypermutation occur?

Answer 19

In the germinal centre of the lymphoid tissue

Question 20

What are centroblasts?

Answer 20

Rapidly proliferating B cells and occupy the dark zone of the germinal centre

Question 21

How many chains are found in Ig’s?

Answer 21

2 heavy chains and 2 light chains (4)

Question 22

What holds the light and heavy chains together and also the 2 heavy chains together?

Answer 22

Inter-molecular disulphide bonds

Question 23

Where is the Fc region?

Answer 23

Tail region of the antibody

Question 24

Where is the Fab Region?

Answer 24

The region containing the antigen-binding site

Question 25

How many antigens can the antibody bind to at once?

Answer 25

2