ON THE RECOGNITION OF THE INDIVIDUAL 

 BY ITAEMOLYTIC ATETTTODS. 



By CHARLES TODD, M.D. 



The discovery and elaboralion nl ihu so-called "Iininunity Reactions" 

 show signs of proving one of the greatest advances made by biological 

 science during recent years, and though discovered originally by 

 bacteriologists in connection with the study of disease, and for a time 

 practically limited to bacteriological research, the significance of these 

 reactions has since overflowed into other fields, and they now constitute 

 our most valuable method for the identification of the proteins. 



For the benefit of those unacquainted with the subject of immunity 

 we may briefly instance one or two concrete examples indicating the 

 nature of some of these rejxctions. If, for example, a rabbit is injected 

 with the white of a hen's egg, it is found that after a certain interval 

 of time the blood-serum of the rabbit which has been so treated has 

 acquired the property of causing a precipitate when added to a solution 

 of hen's egg albumen. This reaction is extraordinarily sensitive — a 

 dilution of even 1 in 100,000 of the albumen still giving a precipitate. 

 It is, moreover, specific, that is to say, the serum, if used in suitable 

 dilutions, only gives a precipitate with the albumen of the hen's egg 

 and not with that of other birds such as ducks, geese etc. 



In the same way if a rabbit is injected with human blood, the 

 serum of the rabbit ac()uires the property of causing a precipitate 

 in solutions of human blood and this constitutes a valuable method of 

 distinguishing human from other blood — a method now in regular use 

 for medico-legal purposes. 



The statement that these reactions are specific requires qualification 

 as this is only the case in certain dilutions. If strong solutions are 

 used the serum causes precipitates, not only in solutions of the blood 



