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Immunology > Antibody Structure & Function > Flashcards

Antibody Structure & Function Flashcards

1
Q

What are the two forms of antibodies?

A

Bound to the surface of the B cell - called a B-cell receptor (BCR)

Soluble - outside the B cell

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2
Q

What role does the BCR plays a B cell activation?

A

Membrane-bound forms help to activate the B cell

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3
Q

Both forms of antibodies are identical with the exception of the ___________________ on the membrane bound BCR form which is removed in order to __________________________

A

Hydrophobic tail;

Allow passage through the membrane for the soluble form

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4
Q

Soluble forms of antibodies bind __________

A

Extracellular pathogens and their products (toxins).

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5
Q

T/F. Soluble forms have the SAME specificity as the BCR.

A

True

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6
Q

When the antibody binds, what do they do to the pathogen?

A

They don’t harm or kill the pathogen, but deliver it to another cell of the immune system which can remove/kill it
- Antigen is marked for opsonization or ADCC by cells with Fc receptors

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7
Q

5 main isotype of antibodies

A

IgA, IgD, IgE, IgG, IgM

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8
Q

In which ways are antibodies isotypes similar and different?

A
  • Isotopes share a common structure and overarching function.
  • Each isotype has unique properties and specific functions
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9
Q

Antibodies are highly specific for a particular _______ while also being highly diverse in ______________

A

Epitope; sequence and function

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10
Q

Antibodies are highly diverse in portions of their sequence that provide ____________

A

Antigen specificity

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11
Q

T/F. Antibodies are highly specific. Each antibody recognizes only a small, specific area on a particular antigen (epitope).

A

True

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12
Q

Antibodies have conserved sequences that allow them to __________

A

Carry out common functions

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13
Q

Which part of the antibody molecule determines specificity?

A

Variable regions

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14
Q

Which part of the antibody molecule determines an anatomic distribution?

A

Ch- Heavy chains

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15
Q

The __________ regions determine isotype/class. Therefore, different isotype have different __________________.,

A

Constant regions; biological activities/functions

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16
Q

Heavy chains gross structure

A

2 H chains are held together by two interchain S-S bonds

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17
Q

Describe the 2 regions of the Heavy chain

A

Variable (Vh)- different for antibodies produced by cells derived from different clones

Constant (Ch)- same for each antibody within an isotype, but different from one isotype to another.

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18
Q

Gross structure of Light Chains and relation to the H chains

A

The L chains are bound to the H chains through a single interchain S-S bond

The L chains do not interact with each other

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19
Q

The (heavy/light) chains within an antibody molecule will be (identical/different).

A

Identical for both heavy and light chains

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20
Q

What are the 2 types of L chains?

A

Kappa and Lambda

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21
Q

Describe the 2 regions in each L chain

A

Variable (VL)- different for antibodies produced by cells derived from different clones

Constant (CL)- same for each type (kappa or lambda)

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22
Q

Variable (V) Region.

A

V region comprises only the first 110 amino acids of the polypeptide chain (remaining sequence is C region).

VL and VH determine antigen specificity

Note: VL=VL; VH=VH; VL NOT = VH

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23
Q

Constant (C) Region

A

The sequence of the constant part of the chains remains relatively unchanged within each isotype.

Ex.

CH for IgM = CH for IgM
CH for IgM NOT = CH for IgG
CL for kappa = CL for kappa
CL for kappa NOT = CL for lambda

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24
Q

The CH portion of the Heavy chain determines ________.

A

Function

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25
Q

The CH region of the H chain is further divided into domains (CH#).

A

IgG, IgA, IgD - have 3 domains CH1-CH3

IgM and IgE - have 4 domains CH1-CH4 (no hinge in these isotypes)

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26
Q

What are the functional regions of the antibody?

A

Fab- each monomer has a part that binds the epitope on the antigen

Fc- A part that crystallizes under certain conditions

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27
Q

Importance of Fab and F(ab’)2.

A

Retain the ability to bind antigen.

- they cannot participate in opsonization or ADCC when treated with the enzymes

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28
Q

Importance of Fc

A

Fc has no ability to bind antigen.
- In the non-digested molecule, this portion serves as the recognition piece for opsonization or ADCC by the Fc receptor on certain cells (ex. Macrophages)

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29
Q

Structure of Fab

A

2 identical pieces

- comprised of VL, CL + VH, CH1 attached through S-S bonds

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30
Q

Structure of Fc.

A

Distal portions of both H chains held together by S-S bonds.

Hinge + CH2, CH3 + CH2, CH3

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31
Q

Papain digestion

A

Cleaves above the hinge region resulting in total of 3 pieces

  • 2 identical Fab
  • 1 Fc
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32
Q

F(ab’)2

A

A single fragment that contain both Fab regions, held together by the S-S bonds in the hinge region

VL, CL and VH, CH1, hinge + VL, CL & VH, CH1, hinge

Note- there is no Fc as the remainder of the CH regions below the hinge are degraded into small fragments

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33
Q

Pepsin digestion

A

Cleaves below the hinge region resulting in 1 piece containing both the Fabs linked together by the S-S bonds in the hinge
- 1 (Fab’)2

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34
Q

Hinge region- structure, function, and located where?

A

Short stretch of amino acids located between CH1 and CH2

Provides flexibility so Fab’s can bind epitope at various angles & distances apart

Only present in isotypes with 3 CH domains- IgG, IgA, IgD

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35
Q

J Chain-

  • Associated with
  • structure
  • required for
A

Associated with the polymeric isotypes- IgM and IgA

1 J chain per polymeric Ig molecule

Required for secretion via the plg receptor (plgR)
- crucial to secretory and mucosal immunity results in Slg

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36
Q

Hypervariable regions

A

Occur in specific locations within the V regions

  • are 8-11 amino acids in length
  • include the amino acids that actually make contact with the epitope
37
Q

Where in the antibody structure are the hypervariable regions found?

A

Occur in specific locations within the V regions

Linear in the sequence, but are adjacent in the 3D folded protein

38
Q

What is the role/function of the hypervariable region?

A

Make contact with the epitope

Hypervariable region aka complementarity determining regions (CDRs)

The 6 CDRs within each Fab form the part of the antibody molecule known as the

  • antibody binding site
  • antibody combining site
39
Q

What are the framework regions?

A

The remaining 80 or so amino acids of the V regions

40
Q

Where in the antibody molecule are the framework regions found?

A

Interspersed between the hypervariable regions

4 FR within each VL and 4 within each Vh

41
Q

Summarize Fine Structure for each monomeric antibody.

A

3 HV regions within each VL and 3 within each VH

6 HV regions per Fab

12 HV per monomeric ab molecule

4 FR within each VL and 4 within each VH

42
Q

Types of L chains

A

Kappa

Lambda

43
Q

Do the different L chains have different roles?

A

No

No known effector function associated with the CL part of the L chain

44
Q

An antibody will have (identical or different) L chains

A

Identical.
Both kappa or both lambda

  • polymeric antibodies will have identical monomers
45
Q

Major types/classes of H chains

A

γ, α, μ, δ, and ε. They define classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE, respectively

46
Q

Which H chains classes have subclasses and what are they?

A

IgG has 4 subclasses: IgG1, IgG2, IgG3, IgG4

IgA has 2 subclasses: IgA1, IgA2

47
Q

What is isotype switching?

A

When a B cell changes which isotype it produces

This allows the immune system to fight the same pathogen at all surfaces or areas of the body

48
Q

Which part of the antibody molecule changes with isotype?

A
  • only the CH portion of the H chain will change
  • no changes to CL, VL, or VH
  • antigen specificity will remain the same
49
Q

T/F antigen specificity depends on isotype.

A

False!

Isotype has NO BEARING on antigen specificity.

Isotype is determined by the C region of the H (CH) & L chains (CL)

VH and VL regions do not change when the isotype changes

50
Q

What part of the antibody determine antigen specificity?

A

VL and VH

51
Q

Which part of the antibody determines function?

A

Constant region of the heavy chain (CH)

52
Q

Which part of the antibody molecule determines anatomic distribution?

A

The CH portion of the antibody molecule

53
Q

When is IgM first made?

A

Starts being produced by a fetus at approximately 20 weeks gestation

First isotype to be produced by B cells upon initial activation by T cells (T-dependent response)

Most predominant isotype in response to T-independent antigens

54
Q

What is a major function of IgM?

A

Best at activating classical complement

Agglutinate pathogens because of effective cross-linking

Note- too large for routine secretion into tissues

55
Q

Where is IgM located in the body?

A

Mostly found in serum, but small amounts can be secreted onto mucosal surfaces (via the pIgR binding to the J chain)- polymer

BCR- monomer

56
Q

What are some of the structural characteristics of IgM?

A

Found in polymeric and monomeric forms

Pentameter&raquo_space;»> hexamer

All forms have a CH4 domain and no hinge

Polymer have J chain which allows secretion onto mucosal surfaces

57
Q

When is IgG first made?

A

4-6 months after birth

- before this crosses the placenta

58
Q

What are the functions of IgG?

A

4 subclasses with varying degrees of functional ability

  • Can activate complement
    IgG3 > IgG1 > IgG2
  • Opsonization
  • Neutralize bacteria, toxins & viruses by preventing their entry into host cells
  • ADCC via Fc receptors
59
Q

Where is IgG located in the body?

A

Most abundant antibody in both tissues and serum

- even though not the most abundant isotype made

60
Q

What are the different subclasses of IgG? Do they have different functions?

A

IgG1, IgG2, IgG3, IgG4

Yes!

Numerical designation correlates with their decreasing concentrations in serum

61
Q

When does IgA first start being made?

A

Made by an infant approximately 4-6 months after birth

- protective for infants who are breast fed (passed from mother)

62
Q

What is a major function of IgA?

A

IgA1- recognizes protein epitope

IgA2- recognizes polysaccharide epitopes

63
Q

Where is IgA located in the body?

A

Found mostly on mucosal tissues and in secretions (tears, saliva, sweat, mucus, colostrum)

IgA1 is predominant in serum (2nd most abundant!)

64
Q

What are the 3 main structural forms of IgA?

A

Serum IgA (sIgA or just IgA)

Polymeric IgA (pIgA) - this is a dimer and is the form of IgA after it leaves the plasma cells, but before it is secreted via the pIgR

Secretory IgA found in secretions (SIgA)- this is always a dimer and is the form after it is secreted across the epithelial barrier by the pIgR

65
Q

What is a unique structural feature of dimeric IgA (can also be found on IgM)?

A

???

66
Q

What is the secretory component? On which structural type of IgA is it found?

A

SIgA - always a dimer and is the form after it is secreted across the epithelial barrier by the pIgR

67
Q

Where is IgE mostly found in the body?

A

Nearly all IgE is bound to the high affinity Fc receptors on mast cells, basophils, and eosinophils

68
Q

When is IgE made?

A

When Fab’s are cross-linked due to antigen binding, the attached cell releases its contents (degranulation)

69
Q

What is the main “healthy” function of IgE?

A

IgE binds to parasites, causing degranulation of attached granulocytes to kill the pathogen
- ADCC

70
Q

What is the main “dysfunctional” response associated with IgE?

A

IgE binds to allergens, causing degranulation of attached granulocytes
- Allergies (Type I Hypersensitivity)

71
Q

When is IgD made?

A

Constitutively expressed on B cells (except when under attack)

?

72
Q

What is the main function of IgD?

A

Allows naive, mature follicular B cells to enter lymphoid follicles

73
Q

If IgD is missing, what might occur with humoral immune responses?

A

Deficiency of humoral immunity

74
Q

Which isotypes are found in polymeric form?

A

IgM and IgA
IgM- pentamer or hexamer
IgA- monomer or dimer

75
Q

Which isotypes cross the placenta?

A

Only IgG

76
Q

Which isotypes activate complement?

A

IgM and IgG

77
Q

Which isotypes participate in ADCC?

A

IgG and IgE

78
Q

Which isotypes are considered opsonins?

A

IgG and IgA

79
Q

Which isotype is the first to be secreted upon activation of the adaptive response?

A

IgM

80
Q

Which isotypes have a J chain?

A

IgM and IgA

81
Q

Which isotypes have a hinge region?

A

IgG, IgA, IgD

82
Q

Which isotype is most abundant in serum?

A

IgG

83
Q

Which isotype(s) is/are associated with mucosal surfaces?

A

IgA and some IgM

84
Q

Which isotype do we make the most of?

A

IgA

85
Q

Which isotype(s) mediate allergic responses?

A

IgE

86
Q

Which isotype(s) is/are only found on naive B cells?

A

IgD

87
Q

Which isotype(s) provides the majority of our protection to parasitic infection?

A

IgE

88
Q

Which isotype(s) has/have a secretory component?

A

IgA

Immunology (6 decks)

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