Negative regulator of the type III secretion system regulon. Acts by disrupting transcriptional activator ExsA self-association and DNA-binding activity in absence of inducing signals (PubMed:12421316, PubMed:19369699, PubMed:20008065, PubMed:23279839). Upon host cell contact, this interaction is disrupted by the anti-antiactivator protein ExsC leading to ExsA activation (PubMed:15225323, PubMed:16980486). Can form homotrimer (PubMed:23279839). Interacts with ExsA; this interaction inhibits ExsA activity (PubMed:12421316, PubMed:19369699, PubMed:20008065). Interacts with ExsC; this interaction dissociates the ExsD-ExsA complex (PubMed:15225323, PubMed:16980486, PubMed:20008065). Absence of ExsD leads to transcription of the type III secretion system (T3SS) regulon derepression.