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MIIM30002 > Lecture 15: Immunoglobulin Structure and Function > Flashcards

Lecture 15: Immunoglobulin Structure and Function Flashcards

1
Q

What happens when a B cell recognises the epitope of an antigen?

A

B cells undergo clonal selection, proliferation and generation of plasma cells which secrete antibody + memory cells which persist to combat next infection

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2
Q

How many different encoded specificities of B cells are there in humans?

A

~10^7 different encoded specificities

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3
Q

What does a given antigen bind to on its specific B cell?

A

surface Ig

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4
Q

What is surface Ig?

A

immunoglobulins which are embedded in the B cell membrane

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5
Q

What is the role of surface Ig?

A

acts as the B cell antigen receptor and can transduce activation signals upon antigen binding

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6
Q

How many copies of surface Ig does each B cell express?

A

multiple copies ~10^5 per cell that bind the same antigen

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7
Q

What is secreted Ig secreted by?

A

activated B cells (plasma cells)

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8
Q

What is the difference between secreted IgG and membrane IgG?

A

secreted IgG has all the same regions as membrane IgG except it has a tail piece and membrane IgG has a cytoplasmic tail + a hydrophobic transmembrane region

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9
Q

What is the structure of immunoglobulins? How are they held together?

A

consists of two heavy and two light chains

chains are held together by disulphide bonds

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10
Q

What is the structure of the Ig domains?

A

~110 amino acids long

2 layers of beta-pleated sheet, 3-5 antiparallel strands

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11
Q

What do inward pointing hydrophobic residues form?

A

a stable core

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12
Q

What are light chains consisted of?

A

two Ig domains

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13
Q

What are heavy chains consisted of?

A

four to five Ig domains (depending on the class of antibody)

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14
Q

What regions do chains have? What is the function of these regions?

A

variable (V) and constant (C) regions
variable - antigen binding
constant - effector function

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15
Q

What is the role of the hinge which links antigen-binding arms?

A

allows flexibility in binding to multiple antigens

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16
Q

What are the two functionally distinct parts of Ig?

A

the Fc part and the Fab part

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17
Q

What is the Fc part of Ig?

A

tail constant region, can be 1 of 5 isotypes

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18
Q

What is the Fab part of Ig?

A

antigen binding, hypervariable (humans can make ~10^12 antibody specificities)

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19
Q

What can the immunoglobulin molecule be readily cleaved into using specific enzymes?

A

functionally distinct fragments

20
Q

How does pepsin cleave Ig?

A

separates both Fab regions from digested Fc region

21
Q

How does papain cleave Ig?

A

releases individual Fab and Fc fragments

22
Q

What is most of the variability between different Igs within? What do these form?

A

three regions of Vh and Vl (hypervariable regions)

these form the antigen binding site

23
Q

How long are hypervariable regions?

A

~10 amino acids in length

24
Q

What are hypervariable regions also known as?

A

CDRs or complementarity determining regions

25
Q

In which CDR region is there the most variability?

A

in the CDR3 region

26
Q

How can immunoglobulins differ in their light chain?

A

either have a kappa or a lambda light chain

the same type of light chain is found on both arms of a single Ig molecule

27
Q

How can immunoglobulins differ in the constant region of their heavy chain?

A 
different class or isotype
e.g. mammals have 5 isotypes: IgA, IgD, IgG, IgE and IgM (designated by Greek letter)
28
Q

What are the properties of IgG?

A

high affinity monomer (bivalent)
major class in secondary response
large hinge region (i.e. flexible arms)

29
Q

What are the properties of IgM?

A 
has extra constant Ig domain
pentamer (10 valency)
Ig oligomerization via J-chain
first Ig produced
high avidity although low affinity
30
Q

What is affinity?

A

the strength of binding of one molecule to another at a single site

31
Q

What is avidity?

A

sum total of the strength of binding between two molecules or cells to one another (where multiple binding sites are involved)

32
Q

Do multimeric Ig have high or low avidity?

Do monomeric Ig have high or low avidity?

A

multimeric Ig have high avidity

monomeric Ig have low avidity

33
Q

What are the properties of IgA?

A

dimer (tetravalent)
major Ig secreted into GI and respiratory tracts
26aa tailpiece links to J-chain

34
Q

What are the properties of IgD?

A

low abundance and mainly cell-associated

35
Q

What are the properties of IgE?

A

involved in response to parasites and allergic reactions
binds to mast cells, induces degranulation
extra C domain

36
Q

Where are the different Ig isotypes distributed?

A 
IgG = serum and lymph
IgM = serum
IgA = serum and secretions (e.g. tears, GI tract)
IgD = serum?
IgE = beneath epithelial surfaces
37
Q

What does the constant region confer?

A

functional specialisation on the antibody

38
Q

What are different classes of Igs distinguished by?

A

the structure of their heavy-chain constant regions

39
Q

What are antibody effector functions?

A

neutralisation, opsonization and complement activation

40
Q

What is the role of the Fab region?

A

involves antigen recognition

direct mechanism

41
Q

What is the role of the Fc region?

A

indirect / targeting mechanisms

42
Q

What is neutralisation (direct mechanism)? -> IgG, IgA, IgM

A

Ab binding to proteins used by viruses or bacteria which are essential for attachment
can block infection

43
Q

What is opsonization (indirect mechanism)? -> IgG1 > IgG3 > IgG4 = IgA

A

coating of pathogen with specific Ab -> binding to phagocytes via FcR -> enhanced phagocytosis of bound particle

44
Q

What is antibody-dependent cell-mediated cytotoxicity (indirect mechanism)? -> IgG1 = IgG3

A

Ab binds to target cell via variable region e.g. virus infection cell
Ab binds to NK cell via FcyRIII receptor (CD16)
cross-linking of FcR triggers NK cell killing

45
Q

What is degranulation and what is it carried out by (indirect mechanism)? -> IgE

A

IgE binds to parasite or allergen and FceR on immune cell
cross-linking of FceR triggers degranulation -> release of inflammatory mediates such as histamine
carried out by eosinophils, basophils and mast cells

46
Q

What is complement activation (indirect mechanism)? -> IgM > IgG1 = IgG3 > IgG2 = IgA

A

Ab binding to component C1q via Fc initiates the classical complement cascade
only IgG and IgM complexes can bind C1q and free Ab does not activate C1q

MIIM30002 (25 decks)

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