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Structure and function of antibodies
Immunoglobulin genes
Immunological assays
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Basic structure of antibodies (immunoglobulins)
Tiselius and Kabat, 1939immunized rabbits with ovalbumin
serum was electrophoresed
some serum was incubated with ovalbulinand also electorphoresed
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Treated with ovalbumin (to absorb antibody)
p. 77
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p. 77
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p. 80
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Immunoglobulin domains
4 (or 5) in heavy chain, 2 in light chain.
Both heavy and light chains have
1 variable domain at the N-terminus
about 110 amino acids each
intrachain disulfide bonds
How are chains held together?
disulfide bonds
noncovalent interactions
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Variable regions
Site of antigen interaction
Hypervariable (CDR; complementarity-determining
region) site of antigen binding
Rest of domain- framework
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Fv fragment (VH and VL)
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Constant-region domains
CH1 and CL
stabilize V regions
contribute to antibody diversity
Hinge
flexibility
Fab and
Fc can move around itpresent in IgG, IgA, IgD
IgE and IgM have no hinge, instead
a fourth C domain
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CH2 has conserved glycosylation sites (some
Ig subclasses have additional sites)
Carbohydrate is sequestered between
domains
Spreads out the CH2; these regions tend
to be biologically active
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Carboxy-terminal domain (CH3 or CH4)
Can be membrane-bound or secreted
Secreted form: hydrophilic tail
Membrane-bound: hydrophilic spacer
transmembrane sequence
cytoplasmic tail
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Immature B cell: mIgM only
Mature B cell that has not seen antigen:
mIgM and mIgD
MemoryB cell: mIgM, mIgG, mIgA, mIgE
Any one of these can be combined with the
same antigen specificity
How??
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Ig isotypes differ in size, protein sequence
and function (p. 91)
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IgG- most common in serum; monomeric
four subclasses
Slight differences in structure; significant
differences in function
p. 92
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IgG1 and IgG3 are most active
Fix complementBind to Fc receptors on phagocytes
opsonization
ADCC
IgG4 binds to Fc receptors; does not fix
complement
IgG2 fixes complement moderately; has
low affinity for Fc rceptors
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IgM
pentamer (or hexamer), so 10 antigen-
binding sites
produced in primary response
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IgA
most common antibody in body- not serum,
but in secretions. Monomer in serum,multimer elsewhere
helps protect portals of entry in body
main protective antibody in breast milk
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p. 93
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IgE
Very low concentration in serum
Binds to Fc receptors on basophils and mast
cells; induces hypersensitivity response
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p. 94
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IgD
Very low concentration in serum
Function of sIgD is not known
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Antibodies are good antigens (p. 95)
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Cytoplasmic part of mIg is very short.
how does it function as an antigen-specific
receptor?
As part of a complex
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The immunoglobulin superfamily
Many proteins have a domain-like structuresimilar to immunoglobulins
These other proteins do not share function
and do not bind antigen
What is the significance of this common
structure?
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p. 98
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Summary of antibody features
Basic structure: two identical heavy chains,two identical light chains
Antigen-binding and effector functions
Membrane-bound and secreted forms
Five heavy-chain isotypes that vary in function,
serum concentration and serum stability
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