WordNet records no synonym and no antonym for reagin. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
an immunoglobulin E that is formed as an antibody against allergens (such as pollen); attaches to cell membranes causing the release of histamine and other substances responsible for the local inflammation characteristic of an allergy
Synonyms, antonyms and the broader and narrower relations for reagin come from Princeton University’s WordNet. Full sources and licences.