THE ORGANIC CONSTITUENTS OF MILK 31 



differ sufficiently from those of human milk to be the detrimental 

 factor in artificial feeding at least after the early days of life. 1 



The Biological Relationship of the Milk-proteins. The relation- 

 ship of proteins is difficult, if not impossible, to establish by chemical 

 methods, but the methods which have been discovered by workers 

 on immunity enable much accurate information upon this point 

 to be obtained. 



Three methods have been used in establishing the relationship 

 of the milk-proteins. 



1. The precipitin method. 



2. The method of complement deviation. 



3. Anaphylactic methods. 



The precipitin method is based upon the fact that if a foreign 

 protein is injected into an animal, the organism reacts and forms a 

 substance capable of precipitating further amounts of the original 

 protein injected. 



For instance, when milk or when solutions of the different 

 proteins of milk or of blood are injected into an animal of another 

 species the tissues of that animal respond to the invasion of the 

 foreign substance by producing another substance which is capable 

 of precipitating or throwing out of solution the invading or injected 

 protein. The precipitation of the foreign substance renders it 

 harmless to the organism, and is the method employed by the body 

 to protect itself against the invasion of proteins which do not 

 harmonise with those of the animal concerned. If the blood of 

 an animal which has been injected with milk or milk proteins is 

 collected, the serum thus obtained has the power, when mixed 

 with a solution of the proteins which were injected, of precipitating 

 these proteins. The reaction thus obtained is visible to the naked 

 eye, and when the precipitate is formed it is known that special 

 substances (precipitins) have been present in the serum. More- 

 over, it has been shown that these precipitins are capable not only 

 of throwing out of solution the substances (antigens) as a result of 

 whose injection the precipitin was formed, but they can also precipi- 

 tate, at least partially, the proteins in the blood or milk of animals 

 having a near relationship. In fact, the strength of the reaction 

 affords proof of the nearness or otherwise of the inter-relationship 

 of different classes of animals. 



The application of this method has shown (i) that the proteins 

 of human and cows' milk are biologically dissimilar, so that the 

 milk proteins must be regarded as ' foreign ' for the human infant ; 

 (2) that in both cows' milk and human milk caseinogen and lact- 

 albumin are separate bodies, since a caseinogen-anti -serum will 

 not react fully with lact-albumin, and vice versa ; and (3) that the 

 albumin of milk and the albumin of the blood are biologically 

 identical, in both cows and women. 



1 Cp. Chap. VI. for work on this latter point. 



