Yup , It's called Non Compatable body chemistry , A well known medical fact .
freza , this refers to blood types as well in context to the Rh Factors , Be they Rh Plus or Rh Negative
Some people do cause me to have an unpleasant reaction.
yes but IT is in your head. I am not aware of an actual physical allergy
I doubt you could be allergic to the person exclusively.