Protein antigens that are recognized by specific B cell antigen receptors are endocytosed and delivered to a vesicular compartment for processing, leading to the association of a linear peptide derived from the protein with a class II MHC molecule that can bind and display the peptide (Fig. 11-9). This peptide is presented on the B cell surface to a helper T cell that was previously activated in the T cell zone when its TCR recognized an identical peptide presented by a dendritic cell that had encountered the same antigen. Because the BCR recognizes an epitope of the native protein with high affinity, specific B cells bind and present the antigen much more efficiently (i.e., at much lower concentrations) than do other B cells not specific for the antigen. This is why B cells specific for an antigen respond preferentially to that antigen, compared with other "bystander" cells. A protein antigen that elicits a T-dependent B cell response therefore makes use of at least two epitopes when activating specific B cells. A surface epitope on the native protein is recognized with high specificity by a B cell, and an internal linear peptide epitope is subsequently released from the protein, binds class II MHC molecules, and is
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