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)