Oleuropein is a chemical compound found in olive leaf from the olive tree together with other closely related compounds such as 10-hydroxyoleuropein, ligstroside, and 10-hydroxyligstroside. All these compounds are tyrosol esters of elenolic acid that are further hydroxylated and glycosylated. Oleuropein and its metabolite hydroxytyrosol have powerful antioxidant activity both in vivo and in vitro and give extra-virgin olive oil its bitter, pungent taste. Oleuropein preparations have been claimed to strengthen the immune system.

Reference:

1. 1)The phenolic compounds of olive oil: structure, biological activity and beneficial effects on human health E. Tripoli, M. Giammanco, G. Tabacchi, D. Di Majo, S. Giammanco, and Maurizio La Guardia. Nutrition Research Reviews 18, 98–112 (2005) DOI: 10.1079/NRR200495
2. Preparation of Antimicrobial Compounds by Hydrolysis of Oleuropein from Green Olives W.M.Walter,Jr.,H.P.Fleming, and J.L.Etchells Appl Microbiol. 1973 November; 26(5): 773–776.
3. The Olive Constituent Oleuropein Exhibits Anti-Ischemic, Antioxidative, and Hypolipidemic Effects in Anesthetized Rabbits Ioanna Andreadou, Efstathios K. Iliodromitis etc.American Society for Nutrition J. Nutr. 136:2213-2219,
4. Discovery of small-molecule HIV-1 fusion and integrase inhibitors oleuropein and hydroxytyrosol: Part I. Integrase inhibition Sylvia Lee-Huang, Philip Lin Huang, Dawei Zhang,etc Biochemical and Biophysical Research Communications Volume 354, Issue 4, 23 March 2007, Pages 872-878