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MIIM30002 - Principles of Immunology > Lecture 5 - Ig Structure > Flashcards

Lecture 5 - Ig Structure Flashcards

1
Q

Describe clonal selection theory

A
  • Any given antigen has a B cell specific for it

* Once a B cell sees the antigen, it proliferates

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2
Q

What are the two forms of Ig?

A
  • surface Ig (detector form)

* secreted Ig (effector form)

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3
Q

What happens after clonal selection?

A

Selection of that B cell
• undergoes proliferation
• starts producing a lot of Ig

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4
Q

Describe the structure of Ig

Draw it

A 
Two version of two different chains:
 • 2x light chains
 • 2x heavy chains
Disulfide bonds holding the chains together
Chains are made up of Ig domains
Variable region (antigen binding)
Constant region (effector function)
Antigen binding arms
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5
Q

What is an Ig domain?

How many does each chain have?

A

A section of the heavy and light chains on the Ig
Light chain: 2 domains
Heavy chains: 4 domains

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6
Q 
Describe the structure of an Ig domain
Describe some key features:
 • stability
 • hydrophobicity 
What are the two types of Ig domains?
How many strands in each?
A

Sandwich:
• two layers of antiparallel Beta Strands (thus, two beta sheets)

• Sheets held together with disulphide bond

  • Very stable
  • Antiparallel
  • Hydrophobic residues pointing inwards

Two types:
• V like: 9 strands
• C like: 7 strands

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7
Q

How many strands in the V-like Ig domains?

A

9 strands

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8
Q

How many strands in the C-like Ig domains?

A

7 strands

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9
Q

What is the Ig superfamily?

Give some examples

A

A family of proteins that aren’t antibodies, but contain domains that are found in antibodies

  • TCRs
  • MHC class II
  • CD4
  • CD8
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10
Q

Where are Ig superfamily proteins generally found?

A
  • Membrane bound
  • Secreted
  • ER, Golgi

i.e., not in the cytosol

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11
Q

Why is it important that Ig-like domains are encoded by a single exon?

A

It can be moved by transposons around the genome

It is for this reason that these domains are so conserved

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12
Q

Give an example of an Ig-like domain in various membrane proteins

A
  • in TCR
  • in CD4
  • in MHC II
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13
Q

Which proteases were commonly used to digest Ig?

A
  • Papain

* Pepsin

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14
Q

Where on Ig does papain cut?

What about pepsin?

A

Papain: Above hinge
Pepsin: below the hinge

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15
Q

Using papain, which two fragments were gotten?

A
  • Above hinge: Fab

* Below hinge: Fc

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16
Q

Which fragments were gotten when Ig was digested with pepsin?

A

• Fab’2
Cut below the hinge

• Below the hinge degrades

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17
Q

What is the antigen binding site made up of?

What is the name for these loops?

A

3 loops from each chain
(Thus 6 loops)

The loops are called:
• CDR: complementarity determining regions
• Hypervariable loops

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18
Q

Where are the hyper variable regions located?

A

Throughout the variable domains of the Ig, but the loops are often at the ends, exposed.

This makes sense, because they are the ones that need to bind

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19
Q

What are the different Ig classes?

What are the corresponding greek letters?

A 
IgA - α
IgD - δ
IgE - ε
IgG - γ
IgM - μ
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20
Q

What is the actual difference between the classes?

A

Different heavy chain

21
Q

What is the further diversity in heavy chains?

A

There are subclasses within each heavy chain class.
For example:
IgG(1-4)

22
Q

What is the diversity in light chains?

A

Two types:
• kappa
• lambda

** Any given B cell will only produce one of these types

23
Q 
Which is the most abundant Ig class in the serum?
Which is the least abundant?
A

IgG

IgE is the least abundant; because it is bound to Mast cells

24
Q

Describe IgE binding to Mast cells

A

IgE Fc region binds to FcR on the mast cells

25
Q

Where is IgA mainly found?

A

Serum and secretions

26
Q

Where is IgM mainly found?

A

Serum

27
Q

Where is IgG mainly found?

A

Serum and lymph

28
Q

Where is IgE mainly found?

A

On mast cells under epithelium

29
Q

Which isotopes are monomers?

What about the others?

A 
Monomers:
 • IgG
 • IgE
 • IgD
Multimers:
IgM:
 • pentamer
IgA:
 • dimer
30
Q

What are some features of IgG?

A
  • Large hinge region
  • High affinity
  • 3 C Ig domains
31
Q

What are some features of IgM?

A
  • 4 C Ig domains
  • Pentamer
  • Low affinity, however, high avidity
  • Stiff hinge region
  • Tail piece (where J chain attaches)
32
Q

What joins pentamers and dimers?

A

J-chain

33
Q

What is a mu chain?

A

The heavy chain of the IgM class

34
Q

What difference does avidity make?

A

When there are more binding sites, there are more chances for binding.

With low avidity molecules, the Ig is either bound or unbound

35
Q

Differentiate between affinity and avidity

A

Affinity: closeness of binding between two molecules

Avidity: sum total of strength of binding at all the sites

36
Q

Why is the tail piece important?

A

This is where the J-chain joins up the 5 IgM molecules to make the pentamer

37
Q

What are the features of IgA?

A
  • Dimer
  • Secreted into mucosa
  • Tail piece + J-chain
  • Secretory component
  • 3 C Ig domains
38
Q

What are some features of IgD?

A
  • Low abundance

* Found on mature B cells

39
Q

What are some features of IgE?

A
  • Involved in allergic reactions & parasitic immunity
  • Binds to mast cells
  • Has an extra C domain (Cε4)
40
Q

Which isotopes have a stiff hinge region?

A

IgM

41
Q

Who described clonal selection theory?

A

McFarlane Burnet

42
Q

Where are disulphide bonds seen in a Ig molecule?

A
  • Between light and heavy chain
  • Within domains
  • Between heavy chains below the hinge region
43
Q

Give the size of light chains and heavy chains in kDa

A

Light chain: 25 kDa

Heavy chain: 50 kDa

44
Q

How long is a single Ig domain?

A

70-110 aa

45
Q

Describe the connection between the two Beta pleated sheets in Ig domains

A

Sandwich of two beta sheets is held together by a di-sulfide bond
Conserved cysteine residues form the bridge

46
Q

Compare the valency of Fab and F(ab’)2

How do you get these two molecules?

A

Fab: monovalent
• digestion with papain

F(ab’)2: divalent
• digestion with pepsin

47
Q

What determines the idiotype of an Ig molecule?

A

The CDRs in the variable domain

48
Q

How many sulfur atoms in a di-sulfide bond?

A

Two

49
Q

Which Ig isotypes have 4 constant Ig domains?

A
  • IgM

* IgE

MIIM30002 - Principles of Immunology (30 decks)

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