Some of the mechanisms proposed for the role of DR in RA.(a) DR molecules provides peptides for binding to other MHC moleculeswhich will trigger T cells that regulate pahogenic T cells. (b) DR- selfpeptide complexes select the T cell repertoire that may have a regulatoryimpact on the activation of pathogenic T cells. (c) DR molecules bindself peptides, which may or may not be derived from joint tissue, that triggerpathogenic T cells. (d) Another possibility is that DR molecules bindspeptides derived from infectious agents that may persist in the joints or giverise to a self cross-reactive T cells. Subsequently the activated T cells couldinduce, modulate or regulate the erosive destruction of the joints.