sensitizing has 3 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is desensitizing.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
rendering an organism sensitive to a serum by a series of injections
making susceptible or sensitive to either physical or emotional stimuli
Synonyms, antonyms and the broader and narrower relations for sensitizing come from Princeton University’s WordNet. Full sources and licences.