A couple of thirsty vampires decide to raid a NHS blood bank after a long drought of their favorite O Negative tipple.
'Can't stand the A and B Positive Groups myself,' one bloodsucker tells his pal, 'always bring me out in hives'.
'You'd better check the blood bag label small print,' his undead chum replies, 'if you don't want an allergic reaction.'
They break into a hospital hematology department and are soon knocking back the samples when the dreaded freak response suddenly rakes place.
'I thought all these vials are supposed to be O Negative,' the squirming vampire screams, 'that's what it says on the damn blood bag.'
'Might be a careless spelling mistake,' his pal replies, 'do you know what the most commonly misspelt blood group is?'
'Don't tell me!'