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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.

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  • $\begingroup$ MHC and B cell receptors/antibodies are totally different things, and recognize completely independent antigens. I think you're mixing up the two. $\endgroup$
    – MattDMo
    Commented Jun 20, 2022 at 0:23
  • $\begingroup$ But doesn't it take a Th cell specific to the same antibody as a B cell to cause the B cell to type switch? And the Th is immune restricted to class II MHC. $\endgroup$
    – Rand
    Commented Jun 20, 2022 at 2:37
  • $\begingroup$ Th cells are not "specific to the same antibody" as B cells because T cells don't care about antibody/BCR recognition sites. B cells are activated by Th cells that recognize the same antigen (a protein, for example), but not the same epitope. BCRs/antibodies and MHC (I or II) have very different epitopes. A B cells Class II doesn't necessarily display the same peptide its BCR recognizes. $\endgroup$
    – MattDMo
    Commented Jun 20, 2022 at 16:18
  • $\begingroup$ Sorry, meant antigen, typo.. how does the Th cell know it recognizes the same antigen as the B cell (or vice versa)? $\endgroup$
    – Rand
    Commented Jun 20, 2022 at 18:04
  • $\begingroup$ Ah, didn't realize that b cells endocytos BCR-bound antigens and present on MHC class 2.. makes sense. $\endgroup$
    – Rand
    Commented Jun 30, 2022 at 3:20

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

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    – tyersome
    Commented Jul 23, 2022 at 0:16

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