Dunno if anyone that reads these boards has a background with Immunology, but here goes. As far as I understand it, heavy chain isotypes can switch on any given mature B cell due to the presence of Switch regions at the 3' of any isotype gene exon. This is regulated largely by cytokines that are secreted upon Ag stimulation. Is it just regular old granulocytes that secrete these cytokines? And in response to what exactly, how does it know which isotype the antibodies should be? Also, how does a placenta get the required antibodies to fight infection if IgG is the only placental transfer isotype. Is this due to chance? (though I highly doubt evolution would leave it at that) Thanks in advance None.