Humoral Immunity and the Generation of Antibody Diversity

Question 1

ANTIBODIES:
What is their structure?

How many domains does the Heavy chain have?

How many domains does the Light chain have?

Which of the domains form the Antigen binding site?

What are the main Ig classes?
→ Which one is most common in the blood?

What are the 2 main functions of Ig’s? How does it do this?

What is the difference between Membrane and Secreted Ab’s?

Answer 1

Tetrameric proteins - 2 identical Heavy chains and 2 identical Light chains, held together by Disulphide bonds

• 3/4 Constant (C) domains and 1 Variable (V) domain
• 1 Constant (C) domain and 1 Variable (V) domain
• VL + VH
• IgM, IgG, IgA, IgD, IgE
  → IgG
• 1. Recognition of an infinite number of Antigens - 3 Hypervariable regions (CDR1-3) in its Fab, which forms 3D loops in order to binds to antigens
     
     2. Effector functions - Block and neutralise antigens through Fab, Opsonisation, Complement activation through Fc
• B cells have membrane-bound Ab’s, which, upon activation, switch to its secreted form through differential splicing of its exons

Question 2

ANTIBODY DIVERSITY:
How do lymphocytes undergo Clonal selection?

In terms of how many clones are used in each immune response, how would you describe the natural immune repsonse?
→ Why is this needed?

What is Somatic Recombination in terms of the Ig gene and what does it allow for?

Answer 2

• Antigen-specific lymphocytes develop in the absence of antigens, and when an antigens enters (infection), the required antigen-specific lymphocyte is selected
• Polyclonal
  → Multiple antigens on organisms, multiple epitopes on each antigen, and more than one antibody may recognise the same epitope
• All Ig genes are INCOMPLETE and are only inherited in segments - Gene segments can be combined to allow for the generation of a many different Igs

Question 3

Generation of Diversity:
What are the gene segments for Heavy and Light chain variable regions?

How are these segments cut up to form different combinations?
→ Is this DNA Joining or RNA Splicing?
→ What is this mediated by?

What does Combinatorial diversity allow for?

What does Junctional diversity allow for?
→ How does it do this?

Answer 3

• • VH encoded in 3 gene segments (V, D, J)
  • VL encoded in 2 gene segments (V, J)
• • Endonucleases cut randomly between the D and J segments to join them together, forming DJ
  • Endonucleases then cut randomly between the V and DJ segments to join them together, forming VDJ
  → DNA Joining
  → RAG (Recombination Activation Genes)
• Millions of combinations to be made
• Increased number of Abs
  → TdT adds random nucleotides to the free ends before joining the segments

Question 4

Allelic Exclusion:
How does this work for the Heavy chains?

How does this work for the Light chains?

Answer 4

• 2 Alleles for Heavy chain, but only one type of heavy chain is made due to Allelic exclusion
  o As soon as one allele is rearranged successfully, it switches off heavy chain rearrangement
• They have Light chain restriction - Each B-cell either makes κ or λ chains