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Clinical Immunology > Antibodies: structure and function > Flashcards

Antibodies: structure and function Flashcards

1
Q

what are antibodies?

A

Antibodies are immunoglobulin (Ig) proteins, produced by plasma cells (differentiated B lymphocytes; aka B cells)

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2
Q

antibodies are part of which immune response?

A

Adaptive ‘humoral’ immune response – specific

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3
Q

Ig structure incorporates features that are…

A

… essential for participation in
the adaptive immune response

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4
Q

Immunoglobulin functions?

A

Specificity where each B cell is programmed to clonally produce an
Ig with specificity for a particular antigen (Ag) and / or epitope

  • Attributed to a defined region (FAB) of the Ig
  • Restricts the Ig to combine with complementary epitopes
  • LARGE variation = large Ig diversity that can react many
    different epitopes
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5
Q

what is specificity attributed to?

A

Attributed to a defined region (FAB) of the Ig

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6
Q

Biological activity is attributed by the…

A

… class of Ig, as well as Ag & epitope specificity defined by the (FC) region

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7
Q

what is biological activity responsible for?

A
  • Complement fixation & activation
  • Passive immunity e.g. crossing placenta
  • Activation of mast cells / basophils (hypersensitivity)
  • Toxin neutralisation (agglutination)
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8
Q

Immunoglobulins (Ig) function as…

A

… antibodies

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9
Q

the antigen (Ag) binding receptors in the B cell surface are secreted by

A

… plasma cells

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10
Q

H chains consist of …

A

… one variable domain
and three constant domains

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11
Q

L chains consist of…

A

one variable domain
and one constant domain

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12
Q

V-domains =

A

specificity

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13
Q

C domains =

A

biological activity

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14
Q

Higher vertebrates produce antibodies and show homology; however …

A

… an immunoglobulin in one species (mouse) is immunogenic in another (rabbit)

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15
Q

Rabbits immunised with mouse Ig will…

A

… produce anti-mouse Ig’s

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16
Q

Subsequent biochemical & serological
have helped distinguish the following
Ab structural characteristics: ???

A
  • Two major L-chain classes, κ and λ
  • κ and λ ratio in humans is 60% κ
  • L-chains are either κ or λ and never one of each
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17
Q
A
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18
Q

how many H-chain classes are there?

A

5

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19
Q

Similarities and differences in the five H-chain classes (isotypes)?

A
  • Differ in peptide sequence, length and CHO (carbohydrate) content
  • Different biological function between types and determine Ig isotype
  • Both H-chain isotypes are identical in any given Ab
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20
Q

what are the five Ig isotypes?

A

IgG
IgA
IgM
Ige
IgD

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21
Q

The five heavy chains?

A

y (gamma)
a (alpha)
u (mu)
e (epsilon)
delta

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22
Q

14

A
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23
Q

Fc (constant) regions provide…

A

… biological activity

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24
Q

Fc (constant) regions provide biological activity while Fab (variable)
regions provide…

A

… epitope specificity

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25
Q

The greatest variability is in the…

A

… N-terminal 110 amino-acids of both the
Light (VL1) and Heavy (VH1)

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26
Q

15

A
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27
Q

Functioin of Complementary Determining Regions (CDRs)?

A

Participate in binding to antigen via epitope complementary structures

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28
Q

There are 3 CDRs for each…

A

… Light (VL1) and Heavy (VH1) chain: CDR1, CDR2 and CDR3

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29
Q

The CDRs are…

A

… spatially separate in primary sequence

30
Q

The CDRs are spatially separate in primary sequence (CDR1 ~aa26-33; CDR2 ~aa48-60 and CDR3 ~aa88-98) come together in…

A

… the secondary β-sheet (Greek key motif), known as the combining site

31
Q

CDRs provide a…

A

… rich diversity in the shape of the combining site

32
Q

Ab:Ag are…

A

… weak integrations

33
Q

Ab:Ag are weak integrations so there should…

A

… be a close fit over a large area where Ab:Ag binding requires both H and L chains

34
Q

Combining sites sometimes …

A

… form a cleft

35
Q

On occasion small hydrophobic molecules (e.g. hapten)…

A

… do not occupy the full combining site yet have sufficient affinity for binding

36
Q

On occasion small hydrophobic
molecules (e.g. hapten) do not
occupy the full combining site yet
have sufficient affinity for binding.

Thus these sites may…

A

… be able to bind diverse epitopes – redundancy

37
Q

Define allotype ?

A

Allotype refers to the genetic variation between individuals for the
same gene at the same loci (alleles)

38
Q

Allotypic differences in immunoglobulins usually involve…

A

… one or two amino-acid changes in the Fc region and generally doesn’t affect Ab:Ag binding .

39
Q

whats an idiotype?

A

refers to the uniqueness of an Ab’s combining site

40
Q

Anti-sera raised to this ‘idiotype’ can then…

A

… block the biological activity of this Ab through competitive interaction at the combining site.

41
Q

What are the functions of an antibody?

A

Agglutination
Activation of other immune components
Neutralisation of toxins
Immobilisation of microbes
Passive immunity

42
Q

What’s agglutination?

A
  • Clumping together
  • Precipitation
  • Promoting phagocytosis
    (opsonising)
43
Q

what happens in neutralisation of toxins?

A

antibodies bind to regions of the toxin responsible for adverse biological effects

44
Q

Immobilisation of microbes alspo has an …

A

… opsonising effect.

45
Q

Passive immunity is present in…

A
  • placenta
  • breast feeding
46
Q

Activation of other immune components…

A
  • Classical complement pathway
  • Causes cell lysis
  • Phagocytosis (opsonising)
  • Activates natural killer cells
    (ADCC)
47
Q

IgG structure?

A

two (κ or λ) L-chains and two γ H-chains
mol.wt 150 kDa (150 000 Da)

48
Q

Where is IgG located?

A

Blood (plasma and serum), lymph, cerebrospinal fluid
(CSF) and peritoneal fluid.

49
Q

WHat are the subclasses of IgG ?

A

IgG1, IgG2(a and b), , IgG3 and IgG4

50
Q

Half lives of IgG ?

A

IgG1, IgG2 and IgG4 – 23 days
IgG3 – 7 days

51
Q

what kind of agent is IgG?

A

IgG is a potent agglutinating agent

52
Q

IgG is a potent agglutinating agent.

What does this mean?

A

readily forms precipitates with multivalent Ag (bacteria etc…)
Precipitant à insoluble Ab:Ag à phagocytosis.

53
Q

Passive immunity

A

All IgG subclasses (except IgG2) pass through placenta

54
Q

What does opsonisation increase the likelihood of?

A

increases the likelihood of phagocytosis via precipitation/agglutination, and phagocyte cell surface.

55
Q

What does ADCC stand for ?

A

Antibody Dependent Cell Cytotoxicity

56
Q

What is ADCC ?

A

where Fab region bind target (virus, bacteria) and protruding Fc
region engages with Fcγ-receptor on cytotoxic immune cell e.g. CD16 on a Natural Killer cell.

57
Q

Antibody Dependent Cell Cytotoxicity process ?

A

1) Antibodies bind antigens on the surface of target cells.
2) NK cell CD16 Fc receptors recognise cell-bound antibodies.
3) Cross-linking of CD16 triggers degranulation into a lytic synapse
4) Tumour cells die by apoptosis.

58
Q

what mediates cell lysis ?

A

Complement activation

59
Q

To neutralise toxins, where does IgG bind?

A

Binds to active part of toxin neutralising the biological
effects – horse IgG given to humans to counteract
snake bite venom

60
Q

to imobilise bacteria, what does IgG interact with?

A

IgG interacts with epitope sites on Ag (e.g. flagella) preventing
movement…. Immobile à agglutination à phagocytosis

61
Q

How does IgG neutralise viruses?

A

Binds to viral Ag on viral coat preventing viral attachment to host cells and thus penetration into cell.

62
Q

IgA structure?

A

two (κ or λ) L-chains and two α H-chains mol.wt 160 kDa (160 000 Da) per monomer.

63
Q

What does IgA exist as?

A

… exists as a dimer (or trimer) linked together with a
J chain; mucosal IgA also has a secretory component

64
Q

where is IgA located?

A

Saliva, sweat, mucus, gastric juice, plasma (low levels) and tears

65
Q

What are the IgA subclasses?

A

IgA1 (93%) and IgA2, both with half lives of 5.5 days

66
Q

What is Plasma IgA ?

A

monomers that are released prior to dimerisation

67
Q

When is secretory IgA formed?

A

formed during transport through mucosa epithelial cells (transcytosis)

68
Q

Dimeric IgA binds to …

A

… poly Ig receptor on basolateral membrane of epithelial cell
(endocytosis)

69
Q

At apical surface, poly
Ig receptor is…

A

… cleaved enzymatically

70
Q

At apical surface, poly
Ig receptor is cleaved
enzymatically.

What is then released?

A

releasing IgA, which remains bound to the poly-Ig receptor (secretory component).

Clinical Immunology (8 decks)

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