noun
Usage: usually capitalized W
Etymology: after August von Wassermann died 1925 German bacteriologist
: a complement-fixing reaction occurring with the serum of syphilitic patients and used as a test for syphilis, being ordinarily made by heating the patient's serum to destroy complement, mixing it with a fortified alcoholic extract of beef heart that is a nonspecific but effective antigen, and adding this mixture to a mixture of washed red blood corpuscles (as of sheep) and a serum (as of rabbit) containing a specific hemolysin for them after which the serum from a syphilitic patient combines with the antigen from the beef heart and absorbs the available complement so that there is no hemolysis while a serum that is not syphilitic causes no reaction in the first mixture and leaves complement free to cause hemolysis when the second mixture is added