ANTIGENS, HAPTENS, ANTIBODIES, ETC. 403 



immunological reactions. Although they are not capable 

 of such great variation in composition as the proteins 

 and polysaccharides there is still a fairly wide range of 

 possible lecithins and kephalins (containing different 

 saturated and uixsaturated acids) and sterols. It seems 

 almost certainly established that they cannot act as 

 antigens alone but that they may do so when mixed 

 with serum ; that is, like most polysaccharides, they are 

 haptens. Synthetic distearyl lecithin was shown to give 

 complement fixation with an antisenim prepared against 

 the compound mixed with pig serum ; cross -reactions 

 with commercial lecithin preparations were also obtained. 

 S}Tithetic and purified lecithins were found to be weaker 

 antigens (when mixed with serum) than crude lecithins. 



The Wassermann and flocculation reactions used in 

 the diagnosis of syphilis employ lipoid " antigens " 

 obtained by alcoholic extraction of heart muscle tissue. 

 Their specific activity with s}q^)hilitic sera appears to 

 depend on a substance, cardiolipin, isolated from the 

 phosphatide fraction. It contains 4-11 per cent, of 

 phosphorus but no nitrogen ; and is isolated as a sodium 

 salt. On saponification it gives 62 per cent, of fatty acids, 

 a non-reducing carbohydrate and phosphoric acid. 

 Glycerol is absent, and the substance is analogous to 

 the carbohydrate containing lipoids of the tubercle 

 bacillus and Lactobacillus acidophilus (see p. 371). 

 Lecithin and cholesterol are also necessary in the " anti- 

 gen " for complement fixation to occur. Other lipoid 

 fractions, fats and fatty acids, also afford reactions 

 which, however, are not specific, occurring with normal 

 as well as syphilitic sera. Bacterial lipoids, like those of 

 plant and animal origin, behave in a similar w^ay with 

 sera, giving the same types of flocculation reaction. 



The sterols, when mixed with pig serum, also seem to 

 be antigenic and give antisera which show complement 

 tixation but not flocculation reactions. Cholest-erol, 



