A clinical association between a specific human leukocyte antigen (HLA) allele and idiosyncratic adverse drug reactions (IADRs) is a strong indication that IADRs are mediated by the adaptive immune system. For example, it is well-established that HLA-B15:02 and HLA-B57:01 are associated with carbamazepine-induced Stevens-Johnson syndrome (SJS)/toxic epidermal necrolysis (TEN) and abacavir-induced hypersensitivity/flucloxacillin-induced liver injury, respectively. Drug-specific T-cells whose response is restricted by specific HLA risk alleles have been detected from IADR patients, also suggesting an adaptive immune pathogenesis. T-cells from carbamazepine SJS/TEN patients are activated by direct pharmacological interaction between carbamazepine and HLA-B15:02 expressed on antigen presenting cells (APCs). Abacavir-specific, HLA-B57:01-restricted T-cells are activated by APCs presenting peptides which are only displayed by the HLA molecule when abacavir is bound during peptide loading. Finally, HLA-B*57:01-restricted activation of T-cells from patients with flucloxacillin-induced liver injury is dependent on processing of drug protein adducts. Based on these observations, it is now possible to utilize blood from healthy drug-naïve volunteers to study the priming of naïve T-cells to drugs. Future development of these methodologies may lead to the development of assays that predict intrinsic immunogenicity of drugs and chemicals at the preclinical stage of drug development.