IMMUNITY REACTIONS 195 



make the application general, if an animal is injected with sub- 

 stances foreign to its species (antigen), e.g., albumin, animal cells, 

 bacteria, toxin, antibodies or immune substances (precipitins, hemol- 

 ysins, agglutinins, antitoxins) are formed in the injected animal. 



Binding of antigens (toxin, bacteria, etc.) by the immune sub- 

 stances (antitoxin, bacteriolysin, etc.) results from combination or a 

 sort of neutralization which may be compared to the neutralization 

 of an acid by a base. P. EHRLICH* was the first to study this neu- 

 tralization quantitatively and showed in the case of diphtheria toxin 

 and its antitoxin, that the saturation did not occur as in the case of 

 a strong acid, e.g., HC1, and a strong base, e.g., KOH, but that the 

 diphtheria toxin must consist of a mixture of more or less acid toxins. 

 We reach this conclusion, not only from the course of the saturation 

 curve, but also from a study of the different poisonous actions pos- 

 sessed by the individual saturation fractions. Though, e.g., the larg- 

 est part of the diphtheria toxin has an acute toxic action, there is a 

 particular fraction, the toxon, which, after two or three weeks, pro- 

 duces paralysis of the extremities that are quite foreign to the toxin. 

 In different cases various indicators are used as a sign of the union 

 between antigen and immune substances. In the case of toxin-antitoxin l 

 we are reduced to the biological proof by animal experiments; the re- 

 duction in the toxicity of the mixtures is determined from the toxic 

 action remaining in them. In the case of hemolysins, the ability to 

 dissolve red blood cells more or less completely is used as a sign. 

 Precipitins are recognized by testing the antigen against various 

 dilutions and determining the greatest dilution at which turbidity 

 can still be recognized. If, e.g., a rabbit has been injected with 

 goat serum a substance develops in the rabbit which precipi- 

 tates goat serum. If we add to goat-rabbit serum which has been 

 placed in a row of test tubes goat serum in a dilution 1/100, 

 1/1000, 1/10000, etc., we shall find a dilution at which merely tur- 

 bidity occurs. In a similar manner agglutinins are tested (in this 

 instance the immune serum is diluted). 



Since the nomenclature is not uniform, a table of the terms in com- 

 mon use is given here. 



Agglutinins change bacteria so that they may even be precipitated by alkali 

 salts (see antigen). 



Amboceptor, see hemolysin. 



Antigens, foreign substances (bacteria, proteins, toxins, etc.) against which 

 specific antisubstances (antibodies) are developed by an animal injected with 

 them (agglutinins, precipitins, antitoxins, etc.). 



Antibodies, immune bodies. 



Antitoxin, specific antibodies which neutralize toxins. 



End piece, see hemolysin. 



1 [JEROME ALEXANDER observed in the ultramicroscope themutual coagulation 

 of diphtheria toxin and antitoxin and tetanus toxin and antitoxin; diphtheria 

 toxin was not precipitated by tetanus antitoxin. Tr.] 



