Structure and function of antibodies

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Structure and function of antibodies Immunoglobu
lin genes Immunological assays
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Basic structure of antibodies (immunoglobulins) T
iselius and Kabat, 1939 immunized rabbits with
ovalbumin serum was electrophoresed some serum
was incubated with ovalbulin and also
electorphoresed
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p. 77
Treated with ovalbumin (to absorb antibody)
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p. 77
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Study of antibody structure Enzyme digests
Reduction and alkylation Using antibodies as
probes Protein sequencing proteins from
patients with multiple myeloma MOPC- mineral-oil
plasmacytoma (this was long before monoclonal
antibody technology!)
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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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(No Transcript)
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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 it present 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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Boxes indicate functional units
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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 Memory B
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
p. 92
Slight differences in structure
significant differences in function
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IgG1 and IgG3 are most active Fix
complement Bind 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 structure similar 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