What if immunotherapy fails and the person needs the medication they're allergic to (pennicillin)? can the immunotherapy be tried again?

Antibiotic allergy: There is no immunotherapy for penicillin or antibiotics. There are desensitization procedures which are short-lived and allow patients to tolerate the antibiotic they are allergic to for a short-period of time. You will need to seek out an allergist who performs these types of procedures since not all allergists do these procedures.

Desensitization: If you need penicillin right away you should get desensitized to it now , the effect is not forever.