Penicillin antibiotics are known to have the highest rate of allergic reactions.
The penicillin allergy results from the formation of an allergen when the beta-lactam ring reacts with a terminal amine on a lysine residue in a polypeptide. Every time a person is exposed to this allergen, their immune system will sometimes cause an hypersensitivity (anaphylactic) reaction (from
http://www.emedexpert.com/facts/amox...te-facts.shtml).