Can Benadryl (diphenhydramine) help a swelling and hive reaction (itching) allergic reaction to penicillin?

Depends. on the reaction, if just hives and itching, yes benadryl (diphenhydramine) can control it in the right dosage, after discontinuing the offending drug, but if more severe symptoms/swelling you need medical attention with the possible addition of epinephrine and probably steroids,