Why Does the Body Reject Generally Healthy Substances?

[dekoi]

Is it common to see a human body rejecting a specific substance only because it is foreign to it -- while at the same time, being generally healthy for the human body?

 

[rockind78]

Common?...I'm not sure, but beta-lactam antibiotics in the context of a serious bacterial infection might qualify, as there are so many people allergic to penicillin.