Transcript The Prototype Immunoglobulin Molecule

Lecture 13
Antigens, Receptors and
Immunoglobulins
Antigenic Determinants (Epitopes)
Receptor-Epitope Concepts
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Most biological systems can be viewed in terms of
interactions between receptors and epitopes (“hand
and glove”).
Specificity of the interaction between the receptor and
epitope is determined by the amino acid sequence of
the receptor (“tailoring the glove”).
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If the glove is too big, the glove falls off.
If the glove is too small, you can’t get your hand into it.
If the glove is missing fingers…
Examples of Receptor-Epitope
Interactions
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Enzymatic reactions
Antigen recognition
Antigen-Antibody reactions
Biological chaperones (stress proteins)
Cellular communications
Cell-mediated cytotoxicity
Cellular homing (MALT, chemotaxis, etc.)
Basic Immunoglobulin Structure
The Prototype Immunoglobulin
Molecule
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Fab
VH
VL
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CH1
CL
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CH2
Fc
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CH3
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Heavy Chains (five
types: 
Light Chains (two
types: and
Fab fragment
Fc fragment
Constant and
variable regions
Carbohydrate
Disulfide linkages
Immunoglobulin G (IgG)
Fab
VH
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VL
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CH1
CL
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CH2
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Fc
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CH3
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2 Heavy and 2 Light chains
2 Fab and 1 Fc fragment
4 Subclasses (IgG1, IgG2, IgG3,
IgG4)
Mol. Wt. 150,000
~70-75% of serum
immunoglobulin.
The major antibody of the
secondary immune response
Change in affinity with time
Immunoglobulin M (IgM)
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J-Chain
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10 Heavy and 10 Light chains
10 Fab and 5 Fc fragments
Mol. Wt. ~900,000
<10% of serum
immunoglobulin.
Single J Chain (15 kDA)
The predominant "early"
antibody
Most primitive immunoglobulin
No change in affinity with time
Immunoglobulin A (IgA)
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Monomeric- Serum IgA
~15-20% of serum
immunoglobulins
2 Heavy and 2 Light
2 Subclasses (IgA1
and IgA2)
Found in serum
Secretory Immunoglobulin A (sIgA)
J-Chain
Secretory Component
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4 heavy chains and 4 light chains (dimeric)
J-Chain and secretory component
The major immunoglobulin of secretions
Not found in serum
Immunoglobulin E (IgE)
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CH4
2 Heavy and 2 Light chains
Mol. Wt. ~190,000
Trace serum protein
Note CH4 region on H chain
Associated with atopic or
anaphylactic
hypersensitivity
May play role in immunity
to helminthic parasites
Immunoglobulin D (IgD)
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2 heavy and 2 light chains
<1% of serum
immunoglobulins
Serves as a membrane
receptor on B lymphocytes
May play role in antigenstimulated lymphocyte
differentiation
Take Home Lessons
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What is the basic immunoglobulin structure?
What are the characteristics or features of the
various fragments of the immunoglobulin
molecule?
How is the specificity of an antibody “defined”
by the amino acid sequence of the
immunoglobulin molecule?
What features distinguish each immunoglobulin
class?