B&I antibodies

Question 1

What are the 4 key aspects of adaptive immunity?

Answer 1

1. Only occurs in higher vertebrates
2. Has memory (secondary response stronger and faster)
3. Affinity of B cells towards antigen increases with time and persistence of antigen (affinity maturation)
4. Each B and T lymphocyte (born with) has different antigen receptors (randomly produced by gene rearrangement)

Question 2

How did adaptive immunity originate?

Answer 2

Transposon inserted into receptor gene, the transposase (RAG1, RAG2) moved away from gene to operate in trans

Question 3

What are recognition sequences (RS)?

Answer 3

Base pair sequences found at ends of gene segments that rearrange (substrate for RAG1 and 2 recombination)

Question 4

What is the function of the immunoglobulin (Ig) protein fold?

Answer 4

Antibodies are formed from repeated Ig domains (barrel of ~110 amino acids)

Question 5

Describe the structure of an Ig molecule (4)

Answer 5

• 2 heavy chains, 2 light chains (joined by disulfide) with a hinge region.
• Light chain carries antigen binding region
• Heavy chain binds to complement etc. for phagocytosis
• IgG, IgM, IgA, IgD, IgE differ by heavy chain

Question 6

Describe IgM (5)

Answer 6

• Default Ig made by naiive B cells
• Low affinity, high avidity
• 10 binding sites (5 joined molecules)
• Activates compliment
• Membrane bound form

Question 7

What is affinity? (2)

Answer 7

“2 lovers in a crowd”

• When the sum of the ATTRACTIVE forces at two surfaces exceeds repulsive forces
• The higher the affinity, the fewer molecules it takes to associate and dissociate slowly

Question 8

What is avidity? (2)

Answer 8

“Velcro”

• Results from multiple affinity contacts
• Strength of binding can be much higher than individual affinities

Question 9

Describe IgG

Answer 9

Activates compliment

*Can transfer across placenta

Question 10

Describe IgA

Answer 10

*Secreted at mucosal surfaces (tears, saliva, breast milk)

Question 11

Describe IgD

Answer 11

Membrane bound form

Question 12

Describe IgE

Answer 12

“Allergy molecule”

*High affinity to mast cells (to produce histamine, can bind to large molecules like pollen)

Question 13

What is complimentarity?

Answer 13

Measure of how well molecules interact - eg. antibody-antigen affinity

Question 14

Describe the antigen binding site (2)

Answer 14

• Amino acid variation is found in 3 regions called complimentary determining regions (CDR), 3 loops that connect strands in H and L chains
• There are 2 identical binding sites

Question 15

Describe recombination in the Ig locus (4)

Answer 15

• RAG1 & RAG2 active in B & T lymphocytes rearrange genes
• by pairing V, D and J segments to create many combinations coding for CDR3
• Results in amino acid diversity in CDR3 loop on antigen receptor
• Light chain has no D segments

Question 16

What is the clonal selection theory? (3)

Answer 16

• One B cell = one antigen specificity
• B cell responds to antigen, mitosis occurs and some cells have higher affinity
• Results in B cells that have high affinity for antigen that remain in lymph nodes ready to act on the antigen

Question 17

Where does immunity (and B cell production) occur? (2)

Answer 17

• The lymph follicles in the lymphatic system

- Some B cells reside in lymph nodes as long-term memory cells