/ Dictionary / Index O opsonin: Meaning and Definition of Find definitions for: op•so•nin Pronunciation: (op'su-nin), [key] — n. Immunol. a constituent of normal or immune blood serum that makes invading bacteria more susceptible to the destructive action of the phagocytes. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: opsonin (Thesaurus) opsonify opsonize Related Content Daily Word Quiz: immense Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language