A person can develop allergy to certain substance if it was in contact with it before. The immune system needs some time (at least several days) to create inadequate diffence against the allergen. This deffence is inadequate because the allergen is a substance that is potentially harmless, but the immune system recognizes it falsely as enemy and trigers its powerful mechanisms. Therefore, when the body comes in contact with the allergen for the first time, no allergic reaction is triggered. Only certain immune mechanisms are trigerred that create several fast-responsive cells as guardians. The next time the allergen comes in contact with the body, these cells multiply fast enough to produce an immense immune response. When the allergy response decreases, some of the fast-responsive cells are left to wait for another contact. Therefore, allergy to certain substance might last for life.
When it’s about allergy to antibiotics, please note that the antibiotics are incoherent group of substances and allergy to certain antibiotic means that you are allergic to that antibiotic. Allergy to antibiotics that are similar in structure might be present, too. However, most of the other antibiotics might probably be used without any consequences.