Any substance (usually foreign) that binds specifically to an antibody or T cell receptor (TCR). Only tiny regions interact with antibody or TCR called antigenic determinants
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Bacterial cell wall
Surface macromolecules secrete toxins
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Eukaryotic cell membrane
Surface macromolecules identify toxins and foreign macromolecules.
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Basic 4-chain structure of antibody
2 identical light chains, 2 identical heavy chains
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Intact IgG + Ag
Cross-linking of Ag -> agglutination
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Pepsin
F(Ab)2 + Ag ->Cross-linking of Ag -> agglutination
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Papain
Fab + Ag -> NO Ag cross-linking -> NO agglutination
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Green & Valentine
Antibodies raised against an ‘artificial antigen’ Dinitrophenol (DNP) group. Synthesized a divalent antigen . Incubated this divalent Ag with antibody at different conc. ratios.
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IgG Molecules
The Fab arm is rotated by as much as 158°, the Fab-Fc angle ranges from 66° to 123°, and the Fab-Fab angle ranges from 115° to 172°. This flexibility would allow the IgG molecules a wider search range for antigens
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Immunoglobulin domains
Share similar globular structure, common ancestral “Ig domain” gene
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Multiple myeloma
Cancer affecting B cells, uncontrolled proliferation of a single mutant B cell, synthesis of large amounts of homogeneous Ab . Malignant B cells mass-produce isolated light chain dimers
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Variability ratio
Number of different AA residues at a given position Frequency of most common AA residue at that position.
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IgG domains
3 hypervariable regions within each VH and VL domain.
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Hypervariable regions of a Fab
The part of antigen in direct contact with the antibody is called the antigenic determinant, or epitope.
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Antigen epitope
Unique Ag binding cleft created by spatial apposition of 6 hypervariable loops (= 6 Complementarity-determining regions; CDRs)3 on VH & 3 on VL
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Roitt’s visualization of Ag binding site
Changing primary AA sequence of each loop -> Huge diversity in shape of binding cleft
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4 Binding forces that contribute to Ab-Ag interaction
Ionic (coulombic), Hydrogen bonding, Hydrophobic interaction, Van der Walls force. Each is reversible, non-covalent and strong in bulk.
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Ab-Ag interaction: hydrogen bonding
The water molecules contribute significantly to the binding energy by creating additional hydrogen bonds.
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Low Affinity Ab
Rapid dissociation of complexes, equilibrium lies to left, Ka is a low (~10-4 mol/l).
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High affinity Ab
Stable complexes, equilibrium lies to right, Ka is high (~10-10 mol/l).
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Association constant
(Ka) = [Ab-Ag Complex] / [Ab] [Ag]
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Affinity
The strength of binding (affinity constant, Ka) between one antigen-binding site on an antibody and one epitope on an antigen).
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Avidity (functional affinity):
The binding strength between antibody and antigen taking into account the multivalent nature of the interaction (i.e. multiple Ag-binding sites, multiple epitopes.
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Avidity
The overall binding strength between antigen and a mixture of antibodies (i.e. in antiserum)
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Immunogen
Can generate an immune response needs to be a macromolecule >~10kDa
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Carrier
Any molecule which when conjugated to a non-immunogenic molecule (e.g. a hapten) makes the latter immunogenic by providing epitopes for helper T-cells which the hapten lacks.
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Antigens
Any foreign macromolecule. Likely to have multiple epitopes targeted by different antibodies generated during IR.
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Epitope
Specific target against which an individual antibody binds.
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Continuous epitope
Linear sequences of amino acids.
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Discontinuous epitopes
Exist only when the protein is folded into a particular conformation - upon denaturation.
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