A 28-year-old woman develops urticaria and angioedema within 15 minutes of receiving intravenous penicillin. A skin prick test with penicilloyl-polylysine is positive. In contrast, a 35-year-old man presents with a maculopapular rash on day 7 of oral amoxicillin therapy, with negative skin prick tests but positive lymphocyte proliferation assay. Which feature best discriminates between these two hypersensitivity reactions?