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

Whale myoglobin Hen egg-white lysozyme

Sequential epitope Nonsequential epitope

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