1) Activator 2) Sensitiser 1) Sensitiser 
• (n.) An agent that sensitizes.
Sen'si·ti`zer noun (Photog.) An agent that sensitizes. « The sensitizer should be poured on the middle of the sheet.» Wilis & Clements (The Platinotype). 
Type: Term Pronunciation: sen′si-tīz′ĕr Definitions: 1. A substance that causes allergy or dermatitis only after alteration (sensitization) of the skin by previous exposure to that substance. Synonyms: antibody
[n] - (chemistry) a substance other than a catalyst that facilitates the start of a catalytic reaction
sensitiser noun (chemistry) a substance other than a catalyst that facilitates the start of a catalytic reaction