Basic structure of antibodies Abs:

1
Basic structure of antibodies (Abs) Antibodies
are heterodimers Chemical and enzymatic
methods revealed basic Ab structure
2
Basic structure of antibodies (immunoglobulins)
Tiselius and Kabat, 1939 immunized rabbits
with ovalbumin (Ag) bled rabbits this
antiserum was electrophoresed some antiserum
was first incubated with ovalbumin and then
electrophoresed anti-ovalbumin Abs were
absorbed from this serum
3
p. 84
4
p. 85
5
p. 86
6
Enzyme digests IgG papain ? 2 Fab Fc IgG
pepsin ? 1 F(ab)2 small peptides Reduction
and alkylation IgG ? 2 H chains 2 L
chains Light chain sequences revealed constant
and variable regions There are 5 major classes
of H chains and 2 types of L chains
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p. 87
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Immunoglobulins (Ig) have multiple domains based
on the Ig fold 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 in each domain Ig-fold
beta-pleated sheet intrachain disulfide
bonds domains separated by switch region
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p. 88
10
p. 88
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How are chains held together? disulfide bonds
noncovalent interactions CDRs
(complementarity-determining regions) in
variable domains bind Ag CDRs also called
hypervariable (hv) regions Rest of domain is
called framework
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p. 91
13
p. 91
14
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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p. 88
17
Carboxy-terminal domain (CH3 or CH4) Can be
membrane-bound or secreted Secreted form has
hydrophilic tail Membrane-bound has hydrophilic
spacer transmembrane sequence and cytoplasmic
tail
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B cells express different classes of mIg at
different developmental stages Immature B
cell mIgM only Mature B cell that has not seen
antigen mIgM and mIgD Memory B cell mIgM,
mIgG, mIgA, or mIgE mIgs expressed sequentially
on a single cell have identical Ag specificity
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Ab-mediated effector functions - Opsonization
is promoted by Ab - Abs activate complement
(C) - Antibody-mediated cell-mediated
cytotoxicity (ADCC) kills cells - Some Abs
can cross epithelieal cells by transcytosis
(IgA)
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p. 97
21
p. 98
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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. 99
26
IgE Very low concentration in serum Binds to Fc
receptors on basophils and mast cells induces
hypersensitivity response
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p. 100
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IgD Very low concentration in serum Function of
sIgD is not known
Table 4-2
(p. 96) summarizes properties and biological
activities of human serum Igs. Opsonization C
activation ADCC Transcytosis (e.g., Ab to
mucosal surfaces, IgG across placenta - an
example of passive immunity)
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p. 101
30
p. 102
31
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. 103
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p. 104
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(No Transcript)
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p. 105
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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 (p. 96)