60?. SCIENCE PROGRESS 



other than purely empirical ones, this claim is based. For this 

 purpose it is necessary to study a little more closely the fact 

 of natural recovery. This is a very remarkable phenomenon 

 when what it implies is considered. When a person " catches " 

 such a disease as typhoid fever, it is reasonable to suppose 

 that no very large number of typhoid bacilli are introduced 

 into his system ; but in the course of the disease these become 

 exceedingly numerous, and can be recovered from his blood 

 in enormous numbers, and yet recovery occurs in 90 per cent. 

 of cases. That is to say, that a person, infected when in good 

 health, cannot prevent a few bacilli multiplying into many, 

 and yet, although weakened by the disease, he is ultimately 

 able to get rid of the larger number. The first suggestion is 

 that the bacilli have exhausted all the nutrient material in the 

 patient's body ; but since they can be readily cultivated on 

 the tissues of a convalescent patient, this view is untenable. 

 The alternative suggestion is that the patient's blood has the 

 power of elaborating some substances which are inimical to 

 the bacteria. 



It appears to be true that the blood has the power of forming 

 anti-bodies to any foreign albuminous substance which is 

 introduced into it. This is the fundamental hypothesis upon 

 which all immunity depends, and it is this which is exploited 

 in vaccine therapy. 



The truth of this has become plain through a large number 

 of experiments, an account of some of which will make the 

 conception clear. If ricin be added to blood, the red corpuscles 

 are agglutinated and destroyed. Ricin has an affinity for the 

 red blood-corpuscles, and is therefore called an " haemotropic " 

 poison. If now ricin be injected in small but increasing doses 

 into an animal, the serum of this animal in course of time 

 develops such properties that, if it be added to ricin, the ricin 

 loses its property of agglutinating red corpuscles. It contains 

 a " ricinotropic " substance, which combines with the ricin, 

 and so forms an inert body incapable of agglutinating red 

 corpuscles. 1 



Again, if the red blood-corpuscles of an animal of one 



species, for instance a goat, be mixed with the serum of an 



animal of another species, for instance a sheep, the goat's 



corpuscles will remain intact. But if goat's corpuscles are 



1 Wright, A Short Treatise on Antityphoid Inoculation. 



