Stratification of chemical sensitizers into reactive mechanism groups. 1) Black: classification generally acknowledged. Grey: possibly ambiguous classification. 2) Indicated stimulations did not pass QC tests and were removed from analysis. Chemical reactivity abbreviations: Add/El: Addition/elimination to carboxylic acid derivatives; Koor: Coordination bonds; MA: Michael addition or 1,4-Addition to a/b unsaturated carbonyl; NAR: nucleophilic aromatic substitution, PreH: Prehapten; Chemical activation (outside the skin); ProH: Prohapten; Enzymatic activation; SB: Schiff’s Base reaction; SN2: bimolecular nucleophilic substitution.