• Penicillin (G) is not an antihistamine. It is rather an antibiotic as it is produced by a microorganism (Penicillium, which is a fungi) and it can destroy other microorganisms or can inhibit the growth (even in low concentration) by interrupting their metabolic process. Hence, it cannot fight antigens or inhibit the production of histamines(a chemical substance which causes allergic reactions in our body) like antihistamines.

The reason is also wrong because Penicillin (G) is only effective against gram-negative bacteria and not gram-positive bacteria as it has a narrow spectrum (range of its action is limited to only gram-negative bacteria that is why its spectrum is said to be narrow, not broad).