For Cohorts 1A and 1B, 16 subjects will receive an initial subcutaneous (SC) 5 mg dose of KLH. Fifteen days later the subjects will receive 0.1 mg ID KLH dose and the response will then be assessed 48 and 120 hours after the initial challenge. The ID KLH challenge will then be repeated 28 days later and the response will be assessed at 48 hours (Cohort 1A, 8 subjects) and at 120 hours (Cohort 1B, 8 subjects) post challenge. For Cohort 2, a repeat challenge of either 2 tuberculin Unit (TU) or 10 TU ID PPD will be administered 28 days apart, to 8 subjects. The first challenge response will be assessed 48 and 120 hours post challenge. The second challenge will be administered 28 days after the first and will be assessed at the same timepoints. The repeat challenge 28 days later will allow an intra-subject analysis and will determine if a subject can be used as their own control. Part C of the study will assess repeat ID challenges of PPD and PBS; the objective will be to characterise the T-cell response to each challenge and the kinetics of that response. For Cohort 1, 6 subjects will receive a 0.1ml ID dose of PBS and another dose 24 hours later. Each challenge will be assessed 48 hours post challenge. Approximately 2 subjects will have an assessment of their normal skin as a control. For Cohort 2, a repeat challenge of either 2TU or 10TU ID PPD will be administered 28 days apart, to 6 subjects. The first challenge response will be assessed 48 post challenge. The second challenge will be administered 28 days after the 1st and will be assessed at the same timepoint. The repeat challenge 28 days later will allow an intra-subject analysis and will determine if a subject can be used as their own control.