170 ANTIGENS AND THE TECHNIC OF SERUM REACTIONS 
and with the adoption of the quantitative serum and spinal fluid pro- 
cedures, it should entirely replace the Wassermann test. 
The Kahn Test^ in the Diagnosis of Syphilis.-— Interest in precipita- 
tion tests for syphilis dates back to some early experiments by Michaelis 
in 1907. This worker observed that precipitates formed on mixing 
syphilitic serum with an antigen consisting of a watery extract of syph- 
ilitic liver. The antigen was similar to that employed by Wassermann, 
Neisser and Bruck in the original Wassermann test. Following the 
discovery of Landsteiner and others that alcoholic extracts of tissue 
organs might be used as antigens, Jacobstahl (1910) announced a pre- 
cipitation method in which an alcoholic extract of syphilitic liver was 
used as an antigen and the precipitates were read by dark-field illumin- 
ation. A long series of precipitation tests has since been developed. 
The better knowii among these are the Sachs-Georgi (1918), Meinicke 
(1918), Dryer and Ward (1921) Kahn (1922) and Vernes (1923) 
reactions. 
The Kahn test involves the use of three reagents: blood serum, antigen 
and physiological salt solution. The antigen consists of a cholestero- 
lized alcoholic extract of powdered heart muscle from which the major 
amount of ether extractives have been previously removed. In per- 
forming the test, the antigen is mixed with salt solution according to 
the required "titer." The resulting mixture is a suspension of lipoid 
particles which have the property of becoming immediately dispersed 
on contact with serum or additional salt solution. The antigen sus- 
pension is mixed with serum in suitable proportions. Instantly a 
a precipitate is formed in syphilitic serum of high potency. If the 
serum is of low potency precipitation may not be completed until the 
mixture has been shaken for three minutes. In the case of non-syphili- 
tic serum no formation of a precipitate takes place. 
The development of the Kahn test was intimately associated with a 
study of several factors which govern the precipitation phenomenon in 
syphilis. 
Concentration of Precipitation Mixture.— Kahn observed in 1921 that 
the dilution with salt solution of mixtures of antigen suspension and 
syphilitic serum delays precipitation. Thus, if the same amount of a 
given antigen-suspension serum mixture, such as 0.2 cc, is pipetted into 
a series of tubes and increasing amounts of saline, for example, in 0.5 cc. 
quantities, are successively added to the tubes, the dela\' in precipita- 
tion will be directly proportional to the amount of saline added, and 
when 2 cc. or more are added, precipitation may be inhibited. This 
observation suggested that for sensitive precipitation reactions in 
syphilis, the serum should be used in an undiluted form, the antigen 
employed should be of high concentration of lipoids, and that a 
1 The writer is deeply indebted to Dr. Kahn for the preparation of, and permission 
to include, this highly important section. 
2 For references and more complete summary see Kahn, R. L.: Serum Diagnosis of 
Syphilis by Precipitation: Baltimore, Williams and Wilkins Company, 1925. 
