How was Jules Bordet awarded the Noble Prize in Physiology or Medicine?

Name: Jules Bordet.
Country: Belgium.
Date of Birth: 13 June 1870.
Died: 6 April 1961.
Education: Free University of Brussels.
Subjects Of Study: Antibody complement hemolysis immune system serological test.
Known as: Immunology.
Noble Prize Year: Nobel Prize for Peace in 1919, his discoveries relating to immunity.