In this paper by Abualrous et al., catalyzed peptide exchange, a process that shapes T cell responses, is reported for 12 different MHC class II variants. Experimental approaches taken by the Freund laboratory are combined with a theoretical description by the Noe group. It is shown that the population of a transient state has predictive value for catalyzed exchange and that MHC class II dynamics correlates with the odds ratio for the autoimmune disease rheumatoid arthritis.