Summer Intro Antibody

8/3/2019 Summer Intro Antibody

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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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Summer Intro Antibody