Lecture 4-7: Structure & Function of Ig (B Cell Focus)

Question 1

Immunogen

Answer 1

A molecule that is capable of eliciting an immune response by an organism’s immune system Substance which induces a specific immune response e.g. hapten-carrier

Question 2

Antigen (Ag)

Answer 2

A molecule that is capable of binding to the product of that immune response Target of immune response e.g. hapten

Question 3

Antigenic Drift

Answer 3

(Slow) change in structure of virus antigen Results in some (otherwise) cross-reactive antibodies failing to bind

Question 4

True or false: A T cell epitope can be linear or conformational.

Answer 4

False, Always linear (peptide)

Question 5

True or false: An Ab epitope can be linear or conformation.

Answer 5

True

Question 6

κ:λ ratio in humans

Answer 6

2:1

Question 7

IgG

Answer 7

Monomer (bivalent) High affinity

Question 8

IgM

Answer 8

Has extra C domain (Cμ4) Pentamer (10 valency) - 5 x μ chains High avidity although low affinity

Question 9

IgA

Answer 9

Dimer (tetravalent) Major Ig secreted into GI and respiratory tracts

Question 10

IgD

Answer 10

Low abundance Mainly cell-associated Highly expressed on naive B cells Not actively secreted

Question 11

IgE

Answer 11

Involved in allergic reactions Immediate-type hypersensitivity Involved in response to parasites Binds to mast cells Induces degranulation Has extra C domain (Cε4)

Question 12

Neutralisation

Answer 12

Binding Ab inhibits/neutralizes pathogenic organism e.g. Ab binds to virus, stops attachment to cell and therefore infection e.g. Ab binds to active site of bacterial toxin Viruses and bacteria bind to cell surface receptors for infection Ab binding to proteins essential for attachment can block infection -neutralisation. Antibody binding to the surface of pathogens may disrupt other processes essential for productive infection e.g. uncoating of viruses

Question 13

Opsonisation (Indirect targeting mechanism)

Answer 13

Enhanced phagocytosis by coating with Ab Occurs in three steps: 1. Coating pathogen with specific Ab 2. Binding to phagocyte via FcR 3. Phagocytosis of bound particle

Question 14

Antibody-Dependent Cell-Mediated Cytotoxicity (Indirect/Targeting Mechanism)

Answer 14

Three steps for ADCC: 1. Ab binds to target cell via variable region e.g. Ab binding virus infected cell 2. Ab binds to Natural Killer (NK) cell via FcγRIII receptor (CD16) 3. Cross-linking of FcR triggers NK cell killing

Question 15

Indirect/Targeting Mechanism - Degranulation

Answer 15

Degranulation (e.g. in eosinophil, mast cells): IgE binds to parasite or allergen IgE binds to FcεR on granulocyte Cross-linking of FcεR triggers degranulation - release of inflammatory mediators e.g. histamine This may not reflect the actual order of events - steps 1 and 2 are probably reversed

Question 16

IgA Transcytosis

Answer 16

Poly-Ig receptor on epithelial cells which sit proximal to the gut binds the J chain (of IgA and IgM, but more commonly IgA)

The ‘secretory component’ includes the IgA, and a bit of the poly-Ig receptor from where it was cleaved off the gut lumen side of the epithelial cell.

Question 17

Complement Activation (C’)

Answer 17

Complement is a set of proteins that promote destruction of pathogens and infected cells

Ab binding to C’ component C1q via Fc initiates “classical” complement cascade (only IgG and IgM complexes can bind C1q)

Ag binding can trigger conformational change in pentameric IgM

Upon Ag Binding, IgG can form hexamers

Free Ab does not activate C1q

Question 18

Functions of C’ (Complement) activation

Answer 18

Recruitment of inflammatory cells and production of inflammatory mediators

Production of opsonins

Deposition of pore-forming proteins

Question 19

Which Ab isotype is best for neutralisation?

Answer 19

IgG, IgA

Question 20

Which Ab isotype response is best for opsonisation?

Answer 20

IgG and to a lesser extent IgA

Question 21

Which Ab isotype response is best for ADCC?

Answer 21

IgG

Question 22

Which Ab isotype response is best for degranulation?

Answer 22

IgE

Question 23

Which Ab isotype response is best for C’ activation?

Answer 23

IgM >> IgG, IgA

Question 24

Allotype

Answer 24

Existence of alleles in Ig loci

e.g. IgG1 will have minor sequence differences between different individuals

Question 25

Idiotype

Answer 25

Variation in the variable region These lead to different specifities of Ig

Question 26

In the Kabat-Wu plot, the region with the largest increase in diversity corresponds to

Answer 26

The region with the largest increase in diversity corresponds exactly to the junction of the V and J segments.

Question 27

During heavy chain gene rearrangement, which rearrangement process occurs first?

Answer 27

D - J rearrangement

Question 28

V(D)J recombinase complex

Answer 28

Recombination-activating genes (RAG-1/RAG-2): involved with breaking/joining DNA Exonucleases: involved in removal of nucleotides and ends generated by RAG’s Terminal deoxynucleotide transferases (TdT): involved in insertion of random, non-template-encoded nucleotides (N-nucleotides/N-regions)

Question 29

Sources of Ig diversity

Answer 29

**Combinatorial diversity** - _Combinatorial joining_ i.e. different combinations of V, D, J for a particular chain and _combinatorial association_ i.e. different combinations of heavy and light chains **Junctional diversity** - contributed by random N-region sequences (estimated ~104). Determined by TdT, endonucleases. **Somatic mutation -** creates diversity as immune response progresses or “matures”. Mostly found in the secondary response

Question 30

Progression of the isotype class switching response

Answer 30

IgM/IgD → IgG → IgA → IgE

Question 31

Terminal deoxynucleotide transferases (TdT)

Answer 31

Involved in insertion of random, non-template-encoded nucleotides N-nucleotides/N-regions N-regions contribute to diversity (i.e. almost any sequence is possible in the N-regions)

Question 32

Activation Induced Cytidine Deaminase (AID)

Answer 32

Induces somatic hypermutation of activated B cells (centroblasts) in germinal centres ## Footnote Converts cytosine to uridine resulting in the introduction of mutations