560 IMMUNITY 



In the course of some weeks' treatment in this way the resulting 

 immunity was of so high a degree that the animals could tolerate 

 on subcutaneous inoculation 400 times the dose originally fatal. 

 Fraser also found in the case of snake venom that rabbits could, 

 by being fed with the poison, be immunised against several times 

 the lethal dose of venom injected into the tissues. In such 

 cases some of the molecules which act as antigens apparently 

 pass through the intestinal wall unchanged. 



By feeding animals with dead cultures of bacteria or with 

 their separated toxins, a degree of immunity may in some cases 

 be gradually developed. But this method is so much less certain 

 in results, and so much more tedious than the others, that it has 

 obtained no practical applications. 



Active immunity of high degree developed by the methods 

 described may be regarded as specific, in the sense explained 

 below. A certain degree of immunity, or rather of increased 

 general resistance of parts of the body (for example, the peri- 

 toneum), can, however, be produced by the injection of various 

 substances — bouillon, blood serum, solution of nuclein, etc. 

 (Issaeff). These agents probably act by producing a local 

 leucocytosis. 



The Properties of the Sera op Highly Immunised 

 Animals. 



Anti-substances and their Specificity. — The fundamental fact 

 in passive immunity, namely, that immunity can be transferred to 

 another animal, shows that the serum in question differs from 

 the serum of a normal animal in containing antagonistic sub- 

 stances to the toxin or bacterium as the case may be, — these 

 being generally spoken of as anti-substances. The development 

 of these bodies, first observed in the case of the injection of 

 toxins, is found to occur when a great many different substances 

 are introduced into the tissues of the living body. We can, in 

 fact, divide organic molecules into two classes — those which give 

 rise to the production of anti-substances, and are thus known as 

 antigens, and those which have not this property. Amongst the 

 former are various toxins, ferments, molecules of tissue cells, 

 bacteria, red corpuscles, etc/ They are all probably of proteid 

 nature, though their true constitution is not known, and none of 

 them have been obtained in a pure condition. Amongst the 

 latter may be placed the various poisons of known constitution, 

 glucosides, alkaloids, etc. We may also state at present that the 

 anti-substance forms a chemical or physical union with the 



