The complement fixation test, the so-called Wassermann reaction, as applied to the diagnosis of syphilis has given rise to an immense amount of work. This has been partly critical. The great bulk of it, however, has been so strongly confirmatory as to leave no doubt as to the great practical importance of the method as a means of clinical diagnosis, provided its limitations are kept carefully in mind.
The method as first devised depends on the principle, formulated by Bordet and Gengou in 1901, of complement fixation by union of a given bacterial substance with the specific "antibodies" found in the blood serum of an infected animal. If, for example, to a properly prepared extract of typhoid bacilli (an antigen) a little blood serum of a rabbit immunized to typhoid be added, and then a little serum of any normal animal for complement, the mixture will combine with the complement