Peyton Manning turned a grown ass woman into a full Mary Katherine Gallagher

It all went down outside "Jimmy Kimmel Live!" in L.A. -- when Peyton and his brother, Cooper Manning, were leaving the studio after the show and stopped to sign an autograph for a fan who was wearing Peyton's jersey. 

The woman absolutely LOST HER MIND -- pumping her fists in sheer excitement after getting #18's signature. 

Can't blame her ... signed Manning jerseys can sometimes sell for $1,000 a pop! 

Nice pull!