Can typed-switched B cells recognize the tertiary structure of antigens? As T helper cells are responsible for isotype switching but Th cells are MHC class 2 restricted and MHC only presents denatured peptides (up to 30 residues, no folding), does that imply that only IgM can recognize tertiary structure? If IgG and the like can, how was that naive b-cell able to present a tertiary structure recognizable by a Th and thus typed-switched.

B cells endocytos BCR-bound antigens and present them on class II MHC. Previously activated Th cells then activate the B cell.