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

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 a.a’s @ surface of Ag  fit into Fab depressions

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

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

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

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

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

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

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

B-cell epitope on HEL Vs.

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

Experiment using BSA carrier with DNP hapten