opsonin
noun
op·so·nin ˈäp-sə-nən 
: any of various proteins (such as antibodies or complement) that bind to foreign particles and cells (such as bacteria) making them more susceptible to the action of phagocytes
Word History
Etymology
Latin opsonare to buy provisions, cater (from Greek opsōnein) + English -in entry 1 — more at oligopsony
First Known Use
1903, in the meaning defined above
Medical Definition
opsonin
noun
op·so·nin ˈäp-sə-nən
: any of various proteins (as complement or antibodies) that bind to foreign particles and microorganisms (as bacteria) making them more susceptible to the action of phagocytes