FIGURE 23-28 Antigen presentation by B cells. B cells bind antigen, even if present at low concentration, to their B-cell receptors, or surface Ig. The immune complex that results is internalized and then delivered to endosomal or lysosomal compartments, where it is destroyed. Peptides liberated from the immune complex, including fragments of the protein antigen, are displayed as class II MHC–peptide complexes at the cell surface. Helper T cells specific for the displayed complex can now provide help to the B cell. This help is MHC restricted and antigen specific.