Antibodies

Question 1

What is the basic structure of an antibody?

Answer 1

• 2 identical light (MW 25,000) chains
• 2 identical heavy (MW 50,000) chains
• Light chain has one variable and one constant domain
• Heavy chain has one variable and 3-4 constant domains
• Variable region/domain is the portion at the tips of the Ab and are the antigen binding sites
• Constant domain is the rest of the Ab after the variable region to the terminal end.
• Each domain is held together by S–S bonds (disulfide)
• There is also a hinge region held together by disulfide bonds!

Question 2

What types of Heavy and Light chains are there?

Answer 2

• 5 kinds of heavy chains (gamma (G), alpha, (A), mu (M), epsilon (E), delta (D)) define the class of antibody to which the molecule belongs & therefore its biological properties
• 2 kinds of light chains (kappa or lambda - the original light chain gene family has duplicated long ago - ex: IgA can be kappa or lambda
• A cell may switch from making IgM to IgA but then the heavy chain changes, the light chain will stay the same!

Question 3

What are the structural features of IgG?

Answer 3

Two light chains, two heavy chains, one subunit, circle on bottom

Question 4

What are the structural features of IgG3?

Answer 4

Like IgG except it has a much larger/longer hinge region (>10 disulfide bonds), better binder of complement due to its hinge region

Question 5

What are the structural features of IgA?

Answer 5

Two individual IgA molecules linked together by their bottoms/constant regions, have a J chain and secretory component linking them together

Question 6

What are the structural features of IgD?

Answer 6

It looks similar to IgG. Has small (one disulfide bond) hinge region. Only found on naive B cells, membrane bound, doesn’t play a role at site of infection.

Question 7

What are the structural features of IgE?

Answer 7

Looks similar to IgM but it is always secreted in monomeric form. It has many carbohydrate units attached. One disulfide bond above CH2 and one disulfide bond below CH2.

Question 8

What is valence when referring to antibody binding?

Answer 8

Valence - refers to the number of antigenic determinants (epitopes) an antibody molecule can theoretically bind.

Question 9

What is the valence of IgG?
What is the valence of IgA?
What is the valence of IgM?

Answer 9

2
4
10 (but in truth 5 due to steric hindrance)

Question 10

What is the valence of Fab?
What is the valence of F(ab’) 2?
What is the valence of VL or VH (isolated)?

Answer 10

1
2
0 - need combination to making binding site!