Rossjohn has provided profound insight into T-cell biology, defining the basis of key immune recognition events by T-cells. He has used structural biology to explain pre-T-cell receptor (TCR) self-association in T-cell development, and how the TCR recognises polymorphic Human Leukocyte Antigen (HLA) molecules. He has unearthed mechanisms of HLA polymorphism impacting on drug and food hypersensitivities, as well as Natural Killer cell receptor recognition. He has pioneered our understanding of lipid-based immunity by NKT-cells, and has demonstrated how vitamin B metabolites are recognized by Mucosal-associated invariant T-cells, thereby defining a completely novel class of antigens recognized by T-cells.