L12 and 13 Diversity in the Immune Response

Question 1

Hb

Answer 1

gene switching within a gene cluster during development

Question 2

Immunoglobulin

Answer 2

rearrangements of contiguous genes/gene segments to create enormous diversity using mechanisms which contravene the “basic dogmas of genetics?

Question 3

Antibody molecule structure

Answer 3

see onenote

1. four polypeptide chains: 2 identical heavy chains and 2 identical light chains
2. 2 antigen binding sites are identical: each formed by N-terminal region of light chain and N-terminal region of heavy chain
3. hinge region formed by two heavy chains

Question 4

Ig variable domain

Answer 4

see onenote

variable domain

• can be split into 3 regions of sequence variability - “complementary determining regions (CDRs)”
• 4 regions of relatively constant sequence termed the framework regions (FR)

Question 5

Ig classess

Answer 5

see onenote

depends on aa sequence of the constant region of the heavy chain

5 classes

Question 6

Three properties of antibodies

Answer 6

1. vast diversity of Ab specificities
2. variable region and a constant region in each chain
3. existence of isotypes with the same antigenic specificity

Question 7

Hypothesis regarding antibody variation

Answer 7

see onenote

1. germ-line theory
2. somatic-variation theory
3. two gene model proposed by Dreyer and Bennett

Question 8

Main stages in B-cell development

Answer 8

Development in bone marrow:

1. stem cell
2. pre-B cell
3. immature naive B cell

circulation through peripheral lymphoid organs:
1. mature naive B cell

all stages occur independently of antigen

when activated by specific foreign antigen and helper T-cells in peripheral lymphoid organs, mature naive B cells proliferate and differentiate into effector or memory cells

Question 9

Once B cells are activated

Answer 9

see onenote

effector cells produce and secrete antibodies with unique antigen-binding site, same as that of original membrane bound antibodies that served as antigen receptors

Question 10

Clonal anergy

Answer 10

antigen may also silence lymphocyte either through apoptotic cell death or induction of refractory state known as clonal anergy

Question 11

Clonal ignorance

Answer 11

if antigen conc. is very low or if affinity of receptor for antigen is low, lymphocyte mat remain unaffected by antigenic encounter

Question 12

Chromosomal locations of Ig genes in human and mouse

Answer 12

see onenote

Question 13

Organisation of Ig germ-line segments in mouse

Answer 13

see onenote

heavy chain families contain additional D for diversity

Question 14

DNA is rearranged during B-cell development

Answer 14

see onenote

Question 15

Mechanism of variable-region DNA rearrangement

Answer 15

see onenote

allelic exclusion, ensure that B cell possesses a single antigenic specificity

the allele is chosen randomly

Question 16

Selection process

Answer 16

see onenote

evolution may tolerate a certain frequency of allelically included dual receptor cells, balancing autoimmune outcome with that of counteracting infection

Question 17

Allelic exclusion

Answer 17

Allelic exclusion is a process by which only one allele of a gene is expressed while the other allele is silenced. For autosomal genes, diploid organisms inherit one copy from each parent.

In B lymphocytes, successful heavy chain gene rearrangement of the genetic material from one chromosome results in the shutting down of rearrangement of genetic material from the second chromosome. If no successful rearrangement occurs, rearrangement of genetic material on the second chromosome takes place. If no successful rearrangement occurs on either chromosome, the cell dies.

Question 18

Gene rearrangement

Answer 18

see onenote slides

Question 19

Junctional flexibilities

Answer 19

see onenote

gives additional variation

Question 20

Hinge region

Answer 20

The hinge region is a flexible amino acid stretch in the central part of the heavy chains of the IgG and IgA immunoglobulin classes, which links these 2 chains by disulfide bonds. It is rich in cysteine and proline amino acids, extremely variable in amino acid sequence, and has no resemblance to any other immunoglobulin region

Question 21

Putative recognition sequence for rearrangement of Ig and T-cell receptor genes

Answer 21

see onenote slides

12-23 rule

Question 22

DNA cleavage by RAG protein

Answer 22

see onenote slides

Question 23

VJD Recombination

Answer 23

see onenote slides

Question 24

SCID

Answer 24

see onenote

Question 25

Somatic hypermutation (SHM)

Answer 25

see onenote

SHM induces point mutations into V region antigen-binding pocket, creating Ig variants with enhanced affinity for particular Ag

Activation induced cytidine deaminase (AID) converts cytidine to uridine to initiate hypermutation process

Question 26

Genetic processes used in generation of antibody diversity

Answer 26

see onenote

Question 27

Four main mechanisms of antibody diversification

Answer 27

1. combinatorial joining of gene segments
2. junctional diversification during gene segment joining
3. combinatorial joining of L and H chains
4. somatic hypermutation

Question 28

Genetic basis of sever combined immune deficiency (SCID)

Answer 28

see onenote

Most commonly due to adenosine deaminase (ADA) deficiency and purine nucleotide phosphorylase (PNP) deficiency - result in accumulation of nucleotide metabolites that are toxic to developing T cells, B cells also somewhat compromised in patients