172 COLLECTED STUDIES IX IMMUNITY. 



The fact that they have suffered a certain quantitative loss in 

 these properties, owing to the extensive manipulation to which they 

 have been subjected, in no way affects their utility for combining ex- 

 periments. In the qualitative demonstration of specific affinity the 

 employment of an excess of receptors answers all requirements. 



In order, furthermore, to meet the objection of a mechanical absorp- 

 tion of the poison by the stromata, exactly similar combining experi- 

 ments were made simultaneously with a blood of the sensitive class, 

 and with one of the insensitive class. As a representative of the 

 former, rabbit blood, which is highly sensitive, w T as used. For the 

 control, guinea-pig blood, which is not dissolved by arachnolysin, 

 was used. The degree of activity of the poison solution before and 

 after binding was measured by means of rabbit blood. 



The stromata sediments derived from each of 40 cc. rabbit blood and guinea- 

 pig blood, are mixed each with 10 cc. of an arachnolysin solution of which 0.025 

 cc. suffice to just completely dissolve 0.05 cc. rabbit blood. The stromata so 

 treated are digested for half an hour in the water-bath at 40 C., being re- 

 peatedly shaken. They are then centrifuged. The decanted fluid from the 

 stromata of guinea-pig blood, like the original material, still completely dis- 

 solves 0.05 cc. rabbit-blood in amounts of 0.025 cc.; the decanted fluid from 

 the rabbit blood stromata, on the other hand, has entirely lost its poisonous 

 action. Even in amounts of 1.0 cc. it is unable to exert the least action on 

 rabbit blood. 



Hence the stromata obtained from the sensitive blood have actually 

 bound the arachnolysin, and this combination must be regarded as a 

 chemical one because the control test with guinea-pig blood shows 

 that the insensitive cell-material exerts no attraction whatever on 

 the arachnolysin. Such behavior, however, is most easily explained 

 by assuming, in accordance with the side-chain theory, the presence 

 of appropriate receptors in the sensitive cells as a prerequisite for 

 the action of the arachnolysin. The natural immunity of certain 

 species of blood will then correspond to an absence of appropriate 

 receptors. We see from this that the distribution of receptors capable 

 of binding arachnolysin, at least so far as the blood is concerned, is 

 not universal throughout the animal kingdom, but confined to certain 

 species. 



While the experiences already mentioned lead us to regard 

 araohnolysin as a poison belonging to the class of toxins, the evidence 

 will be made absolutely conclusive by demonstrating the ability of the 

 poison to produce antitoxin, the most important criterion for the 



