Lecture 4 - Immunoglobulins

Question 1

How are Antigens recognised by B and T Lymphocytes?

Answer 1

• B-cell Receptor (Membrane Inserted) and Antibodies (Secreted Form)
• T-cell Receptor (Membrane Inserted), which cannot recognise naked antigens

Question 2

How does antigen-binding signal into the B-cell?

Answer 2

• Signalling domain - heterodimer of IgB/Iga which signals into cell following conformational change in BCR associated with antigen-binding

Question 3

Define the Terms:
(i) Epitope
(ii) Paratope

Answer 3

(i) Region of Antibody recognised by BCR/Antibody/TCR
(ii) Part of BCR/Antibody/TCR that recognises Epitope

Question 4

Describe proteolytic cleavage of Antibodies in terms of:
(i) Enzymes used
(ii) Fragments Generated

Answer 4

• Papain - yields two Fab and one Fc fragment
• Pepsin - yields one F(ab)2 and multiple pFc fragments

Question 5

Describe the General Structure of the Variable/Constant domains of an antibody in terms of:
(i) chain length
(ii) Secondary Structure
(iii) Bonding involved
(iv) Structural Motifs

Answer 5

(i) approx. 100aa in length
(ii) Each domain contains two B-sheet structures, each formed from multiple B-strands (extended conformation)
(iii) Internal Disulphide bridge stabilises domain fold
(iv) Two Antiparallel B-sheets form the B-barrel structure, which is known as the immunoglobulin fold

Question 6

What are the roles of the Immunoglobulin Superfamily?

(2 Points)

Answer 6

• Antigen Binding (BCR, TCR, Antibodies)
• DNA Binding

Question 7

What are the Hypervariable (HV) Regions?
Where are they found?

Answer 7

• Regions of Heavy/Light Chain Variable regions which differ between antibody/receptor species, conferring specificity of binding
• Present within Hypervariable loops, with 3 HV regions facing in same direction to form half the Antigen-Binding site (ABS)

Question 8

What are the Framework (FR) Regions?

Answer 8

Form the B-sheets and provide structural framework for variable regions

Question 9

How is the Antigen-Binding Site (ABS) Formed?

Answer 9

Co-operation between the Hypervariable loops of both the Heavy and Light Chains forms a single Antigen-Binding Site

Question 10

Compare (i) Affinity and (ii) Avidity

Answer 10

(i) Affinity - strength of binding between one ABS and an epitope (determines rate of antigen dissociation from ABS)
(ii) Avidity - effective binding strength of entire antibody (determines dissociation rate of entire antibody)

Question 11

State the Function of Antibodies in an immune response

(2 Points)

Answer 11

• Bind Specifically to molecules from pathogen (secreted or on surface)
• Once bound, Recruit Other cells/Molecules to destroy the pathogen

Question 12

How is Variability generated in the Variable regions of heavy/light chain genes?

(2 Points)

Answer 12

Combinatorial Diversity - different combinations of heavy and light chains V-regions in single ABS generates variability

Somatic Recombination - second form of variability produced by rearrangement of small segment of Ig-encoding DNA

Question 13

Define the two types of Light Chain, and the Functional Difference Between them

(2 Points)

Answer 13

• Lambda/Kappa
• No Functional Difference

Question 14

How many classes of Heavy Chain are there, and how do they influence the antibody produced?

(2 Points)

Answer 14

• 5 Classes
• Structural Differences exist between them, which determines the Antibody Isotype Produced

Question 15

State the 5 Antibody Isotypes

Answer 15

IgM - produced first
IgG
IgA
IgD
IgE

Question 16

Give 3 Examples of Effector Functions carried out by Heavy Chain Constant regions

Answer 16

1. Can be Recognised by Fc receptors to facilitate phagocytosis
2. Can Activate the complement pathway by binding to certain components
3. Can deliver Antibodies to specific compartments by binding to receptors (e.g., Foetal blood via Placenta)

Question 17

What Antibody Isotypes can form High Order Polymers?

What are the two components Required?

Answer 17

• IgM (Pentamer), IgA (Dimer)

1. 18 aa Tailpiece region in C-region containing a Cys residue
2. J Chain - 15Kd Polypeptide which links monomers together via their Tailpiece region

Question 18

Why are High Order Polymers Important?

(2 Points)

Answer 18

• Important for binding to repetitive epitopes e.g., bacterial cell wall polysaccharides
• May be required for transport through epithelium e.g., IgA