SUPERANTIGEN STEPTOCOCCAL PYROGENIC EXOTOXIN A1 IS RECOGNISED BY MHC CLASSII MOLECULES AND T-CELL ...SUPERANTIGEN STEPTOCOCCAL PYROGENIC EXOTOXIN A1 IS RECOGNISED BY MHC CLASSII MOLECULES AND T-CELL RECEPTORS. THE ZINC ION HAS BEEN SHOWN TO HAVE A ROLE AS A SECOND HIGH AFFINITY MHC CLASS II BINDING SITE AND IN THE THERMOSTABILITY OF SUPERANTIGENS.