B Cell Membrane Proteins Costimulatory molecules>Interact with T cell membrane molecules to enhance activation ·B7 Resting mature B cells ·Low levels B7 ·Poor APC ■Activated B cells ·High levels of B7 Very efficient APC ■CD40 Critical role in isotype switching .Interacts with CD154 (CD40L or CD40 Ligand)on T cells Human X-linked hyper-IgM syndrome Boys with a mutation in CD40 ligand gene(either not expressed or nonfunctional)make only IgM Ab-cannot switch to any other isotypeB Cell Membrane Proteins ◼ Costimulatory molecules → Interact with T cell membrane molecules to enhance activation ◼ B7 ◼ Resting mature B cells ◼ Low levels B7 ◼ Poor APC ◼ Activated B cells ◼ High levels of B7 ◼ Very efficient APC ◼ CD40 ◼ Critical role in isotype switching ◼ Interacts with CD154 (CD40L or CD40 Ligand) on T cells ◼ Human X-linked hyper-IgM syndrome ◼ Boys with a mutation in CD40 ligand gene (either not expressed or nonfunctional) make only IgM Ab –cannot switch to any other isotype