Ch. 4b
Basic structure of antibodies (Ab’s):
Antibodies are heterodimers
Chemical and enzymatic methods revealed
basic Ab structure
Ch. 4b
Basic structure of antibodies (immunoglobulins):
Tiselius and Kabat, 1939immunized rabbits with ovalbumin (Ag)
bled rabbits; this antiserum was electrophoresed
some antiserum was first incubated with ovalbumin and then electrophoresed;
anti-ovalbumin Ab’s were “absorbed” from this serum
Ch. 4b
p. 84
Ch. 4b
p. 85
Ch. 4b
p. 86
Ch. 4b
Enzyme digests IgG + papain 2 Fab + FcIgG + pepsin 1 F(ab’)2 + small
peptides
Reduction and alkylationIgG 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
Ch. 4b
p. 87
Ch. 4b
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 domainIg-fold: beta-pleated sheetintrachain disulfide bondsdomains separated by “switch” region
Ch. 4b
p. 88
Ch. 4b
p. 88
Ch. 4b
How are chains held together? disulfide bonds noncovalent interactions
CDR’s (complementarity-determining regions)
in variable domains bind Ag
CDR’s also called hypervariable (hv) regions
Rest of domain is called “framework”
Ch. 4b
p. 91
Ch. 4b
p. 91
Ch. 4b
Constant-region domains
CH1 and CLstabilize V regionscontribute to antibody diversity
HingeflexibilityFab and Fc can move around itpresent in IgG, IgA, IgDIgE and IgM have no hinge, instead
a fourth C domain
Ch. 4b
CH2 has conserved glycosylation sites (someIg subclasses have additional sites)
Carbohydrate is sequestered betweendomains
“Spreads out” the CH2; these regions tendto be biologically active
Ch. 4b p. 88
Ch. 4b
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
Ch. 4b
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
mIg’s expressed sequentially on a single cell have identical Ag specificity
Ch. 4b
Ab-mediated effector functions:
- Opsonization is promoted by Ab
- Ab’s activate complement (C)
- Antibody-mediated cell-mediated cytotoxicity
(ADCC) kills cells
- Some Ab’s can cross epithelieal cells by transcytosis (IgA)
Ch. 4b
p. 97
Ch. 4b
p. 98
Ch. 4b
IgG1 and IgG3 are most active
Fix complementBind to Fc receptors on phagocytes
opsonizationADCC
IgG4 binds to Fc receptors; does not fixcomplement
IgG2 fixes complement moderately; haslow affinity for Fc rceptors
Ch. 4b
IgMpentamer (or hexamer), so 10 antigen-binding sites
produced in primary response
Ch. 4b
IgAmost 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
Ch. 4b
p. 99
Ch. 4b
IgE
Very low concentration in serum
Binds to Fc receptors on basophils and mastcells; induces hypersensitivity response
Ch. 4b
p. 100
Ch. 4b
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 Ig’s.
Opsonization; C activation; ADCC; Transcytosis (e.g., Ab to mucosal surfaces, IgG across placenta
- an example of passive immunity)
Ch. 4b
p. 101
Ch. 4b
p. 102
Ch. 4b
The immunoglobulin superfamily
Many proteins have a domain-like structuresimilar to immunoglobulins
These other proteins do not share functionand do not bind antigen
What is the significance of this commonstructure?
Ch. 4bp. 103
Ch. 4b
p. 104
Ch. 4b
Ch. 4b
p. 105
Ch. 4b
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)