100 Messrs. J. A. Gunn and K. St. A. Heathcote. 



of immunised and non-immunised animals by ricin. The result seems to 

 be one of considerable importance, because it shows that whereas the heart 

 and intestine become less susceptible to cobra venom in the process of 

 immunisation the red blood corpuscles actually become more sensitive. 

 The most probable reason for this difference is the one suggested in the 

 introduction to this paper, namely, that the red cells, having lost their 

 nuclei, and being no longer living cells in the ordinary sense of the word, 

 cannot respond to the action of repeated doses of the venom in the same 

 way as nucleated cells. 



Summary and Conclusions. 



(a) Natural Imriiunity. — The minimum lethal dose of cobra venom for the 

 cat is twenty times that for the rabbit (by subcutaneous injection per 

 kilogramme). When the excised hearts of rabbits and cats are artificially 

 perfused with Locke's solution so as to remove the serum, it is found that it 

 requires at least four times as strong a solution of venom to arrest the cat's 

 heart as is required to arrest the rabbit's heart. Similarly the isolated 

 intestine of the cat can withstand the toxic action of higher concentration of 

 venom than can the isolated intestine of the rabbit. The natural immunity 

 of the cat to cobra venom is therefore, in part at least, due to a cellular 

 immunity of the tissues of this animal. No such cellular immunity is 

 displayed by the red-blood corpuscles, those of the cat being actually more 

 sensitive than those of the rabbit to the hsemolytic action of the venom. 



(b) Acquired Immunity. — When a rabbit is immunised to cobra venom, the 

 isolated heart and intestine, perfused with Locke's solution so as to remove the 

 serum, withstand higher concentrations of venom than the heart or intestine 

 of a normal unimmunised rabbit. In the process of acquired immunity, 

 therefore, some of the tissues at all events develop a cellular immunity, apart 

 from the antitoxin circulating in the serum. 



In the rabbit immunised to cobra venom the red blood corpuscles, freed from 

 serum, become, on the other hand, more sensitive to the haemolytic action of 

 the venom. At the stage of immunity which has been examined, they display 

 no cellular immunity. When a rabbit is immunised to ricin, the red cells 

 also become more sensitive to the agglutinating action of this toxin. 



It is clear, therefore, that neither in natural nor in acquired immunity can 

 the red blood corpuscles be taken as a reliable index of cellular immunity. 

 This difference between them and the other tissues examined is, it is suggested, 

 due to the fact that the structure and life history of the red blood corpuscles 

 is different from that of the other cells of the body. 



While other results may occur with other toxins, and possibly even with 



