The Human Leukocyte Antigen (HLA) class I and II molecules are cell surface glycoproteins that bind peptides derived from the breakdown of intracellular (class I) and extracellular (class II) proteins and display them at the cell surface for immune surveillance. Whilst peptides derived from self-proteins are generally ignored, novel peptides such as those derived from invading pathogens may stimulate immune responses from circulating T cells. Interestingly a subset of adverse drug reactions (ADRs) are T cell mediated and appear to be caused by HLA-restricted presentation of small molecule drugs or drug induced ligands. The genes encoding the classical HLA molecules are the most polymorphic of the human genome and more than 8000 functional HLA class I, across 3 gene loci, are expressed in the human population. Polymorphisms of the HLA predominantly map to the peptide-binding cleft, altering the array of peptides (the immunopeptidome) that can be presented based on their ability to interact with the cleft residues. Similarly, certain ADRs are strongly associated with specific HLA alleles, suggesting that these polymorphisms also impact drug interactions and the stimulation of drug-induced immune responses. To understand the nature of the interactions between HLA, drug and peptide, we have used LC-MS to interrogate HLA ligands isolated from untreated and drug treated cells across several HLA associated ADRs, investigating both covalent and non-covalent interactions between the causative drug and HLA-peptide. Paired with functional analysis of drug responsive T cells, these investigations suggest multiple modes of HLA-drug-peptide interaction, dependent on the structure of both HLA and drug.