W3L5 - Antibody Structure

Question 1

What is an Antibody?

Answer 1

Y shaped protein consisting of 2 identical heavy chains and 2 identical light chains
- end of y region recognises epitope on antigen
- antigen binding sites on variable region of single Ig molecule are identical
Expressed on naive B cells (hasn’t encountered antigen) - both IgM and IgD
Secreted by plasma cells (B cell lineage) and expressed on memory B cells

Question 2

Immunoglobulin Structure

Answer 2

Heavy chain 
Light chain 
- two types possible, kappa or lambda 
- either 2 kappa or 2 lambda light chains
- variation in Cl region of light chain
Constant regions
- same for each class
- heavy chain constant regions (Ch)
- Ch1 to 3 in IgG, IgA and IgD
- Ch1 to 4 in IgM, IgE
- light chain constant regions (Cl)
Variable regions
- antigen binding sites
- variable light chain regions (Vl)
- variable heavy chain regions (Vh)

Question 3

Across the 5 classes of antibodies, the constant region varies according to?

Answer 3

Protein sequences
Carb composition
Size

Question 4

Immunoglobulin Fragments

Answer 4

Enzymatic digestion with papain
- produces two Fab fragments plus Fc portion
Enzymatic digestion with pepsin
- produces one F(ab’)2 fragment
Reduction of disulphide bonds
- produces two separate light and heavy chains

Question 5

Immunoglobulin Subclasses

Answer 5

Only IgG and IgA have subclasses in humans
IgG
- IgG1, 2, 3 and 4
- occur due to variations in number of disulphide bonds in heavy chains
- vary in concentration and function
IgA
- IgA1 and 2
- subclass A1 predominates
- relative proportions vary according to tissue

Question 6

Immunoglobulins as Immunogens

Answer 6

Immunoglobulins contain epitopes that can induce an immune response
3 types:
1. Isotypic - epitopes on constant regions of both heavy and light chains that are the same within a species
2. Allotypic - epitopes on constant region of both heavy and light chains that vary between individuals of same species
3. Idiotypic - epitopes on variable region of both heavy and light chains

Question 7

What is Avidity?

Answer 7

The overall strength of binding between an antigen and an antibody molecule

Question 8

What is Affinity?

Answer 8

The strength of binding between one antibody binding site and one epitope

Question 9

B cells and Antibody Production

Answer 9

Upon primary exposure, naive mature B cell differentiate into either a plasma cell or memory B cell
Plasma cells produce/secrete antibodies
- production of IgG, IgA and IgE require T cell help
- IgM can be produced by B cells independently
Memory cells express antibody on their surface but don’t produce antibodies until reactivated by their specific antigen upon re-exposure
- long lived memory B cells produce either IgG, IgA or IgE
- short lived memory cells only produce IgM

Question 10

Which two human immunoglobulins have an extra fold domain in their heavy chains?

Answer 10

IgM and IgE

This extra fold domain replaces the hinge region