  urlLink Penicillin allergy: The most common drug allergy : "You aren't born allergic to penicillin, but you can develop an allergy to the drug once you've been exposed to it. After that, re-exposure to penicillin or related antibiotics can trigger an allergic reaction. Allergic reactions occur because your immune system responds to the drug as if it were a harmful substance instead of a helpful remedy. For reasons not fully understood, your body creates antibodies called immunoglobulin to attack the medication. " 
