Delayed hypersensitivity reaction to penicillin can be lost (and gained): Skin test resensitization following...

We present an interesting case that demonstrated that delayed beta-lactam hypersensitivity can be lost, as demonstrated by both the initial skin test and challenge, and gained, as proven by a repeat delayed skin test positivity 4 weeks after the initial evaluation in this case. We postulate that positivity over the intradermal site following the completion of prolonged challenge occurs due to the priming and resensitization of circulating T cells and the homing of these drug-specific T cells to the skin (sites of intradermal testing) where the penicillin antigens are still present. Of note, post-inflammatory hyperpigmentation following skin tests is rare, as typical wheals and flare reactions associated with skin testing resolve completely. Thus, we postulate that the postinflammatory hyperpigmentation that is seen in this case may be representative of T cell mechanisms similar to that of a fixed drug eruption.