1. Epitopes:
Lymphocytes recog. specific sites on molecules  Ag determinant/epitope
With some Ag, B cells recog 1 part; T cells another
The recognition process differs between B’s and T’s:
B cells recog soluble Ag; to highly exposed sites
T cells recog Ag ONLY in assoc with the peptides of MHC self Ag

2. B cell epitopes determined by the nature of the Fab site
epitope can be no larger than the Ag-binding site and binding is determined by the a.a. seq in Fab
there is complementarity btwn a.a’s of Ag and Ab
Usually hydophilic surface of Ag  fit into Fab depressions

3. Interactions of globular proteins with antibody
Complementary binding of aa’s on AG with aa’s of Ab
Shape of epitope of globular proteins determined by 3° structure

4. View of interacting surfaces – contacting a.a.’s shown
in red; protruding glutamines in white on Ag
Typical binding of globular proteins

5. Binding of smaller ligands
Smaller ligands – i.e., small oligonucleotides, peptides, and haptens can bind in smaller crevices within Fab region
In both cases, Ag-binding takes place at the V regions of the antibody

6. Inherent features of globular proteins
Sequential epitopes elicit Ab attachment in or out of native protein
Nonsequential epitopes are a.a. sequences spaced apart from one another in the 1° structure BUT are brought close together by 3° folding of the protein
The antigenicity of these epitopes depend on the conformation of the protein remaining intact

7. Hen egg-white lysozyme
Nonsequential epitope
Sequential epitope
Whale myoglobin
Hen egg-white lysozyme

8. T Cell Epitopes T cells do not recognize native Ag
∴conformational changes to proteins does not affect their role as T cell epitope!
They only recognize Ag that has been processed and presented with MHC molecules
This process requires APC’s

9. Ag’s form tri-molecular bonds with TCR and MHC
T cell epitopes must bind with both MHC and TCR molecules
Ag-processing is required
Epitopes recognized are usually internal peptides of the protein

10. B-cell epitope on HEL
Vs.

11. Haptens
Haptens = small, organic molecules that are antigenic, but not immunogenic
When haptens bind to a “carrier” molecule (producing a hapten-carrier conjugate), they can become immunogenic
Antibodies can be formed for:
Hapten epitope
Carrier epitope
Combinations of hapten and carrier

12. Experiment using BSA carrier with DNP hapten