38 MILK AND ITS HYGIENIC RELATIONS 



into the chemical composition of the milk proteins were made 

 for the most part in the last thirty years of the nineteenth century. 

 Several observers (cp. Eugling and Menozzi and Musso) believed, 

 as a result of chemical analysis and methods of precipitation, 

 that the albumin of milk and of blood were identical. A little 

 later the idea seems to have been abandoned with some emphasis, 

 probably owing to the rise of the physico-chemical school of physio- 

 logists. The difficulties of establishing the identity of any two 

 proteins by purely chemical means are manifestly immense, and 

 the relationship of the proteins in milk and blood has been investi- 

 gated in the present century by other methods. These methods 

 have been evolved by workers on immunity, and have led to 

 the discovery of most interesting facts concerning the proteins 

 of milk. 



These facts are of importance in relation to the transference of 

 immunity by suckling from mother to child, and will be considered 

 in some detail. 



The first step was made by Bordet (1899), who showed that if 

 an animal was injected with milk of another species, the serum 

 of this animal, when withdrawn from the body, was found to have 

 acquired the property of precipitating the milk of the species which 

 had been used for injecting. Such a serum is known as a lacto- 

 serum, and the reaction as the ' precipitin ' reaction. Practically, 

 the phenomenon means that when a foreign protein is injected 

 into the body, a reaction occurs on the part of the organism which 

 enables it to throw this protein out of solution should occasion 

 arise. This occasion is made to arise artificially by removing the 

 serum and mixing it in a test-tube with some of the protein originally 

 used. The substances which are capable of giving rise to this 

 reaction on the part of the body are known as ' antigens.' The 

 reaction is not absolutely specific for the one species, but may occur 

 with the protein of a closely related species. For the present purpose 

 the complication of allied species need not be considered, since 

 there is no question that the proteins of the bovine and of the 

 human species are ' foreign ' to one another. This specificity of 

 human and cows' milk was shown by Rosenau and Anderson, 

 using the method of anaphylaxis, and has been confirmed by 

 numerous investigators. 



Hamburger (1901) showed that an anti-serum for ox blood 

 would also precipitate milk, and that a lacto-serum for cows' milk 

 would produce a precipitation with ox blood-serum. These observa- 

 tions showed a very close relationship between the proteins of blood 

 and those of milk. Hamburger carried his observations yet further, 

 and found that the proteins of milk could be differentiated ; a lacto- 

 serum prepared by the injection of caseinogen will precipitate 

 caseinogen, but not lact-albumin, and vice versa. All the results 

 were not in complete accordance but they were sufficiently 

 definite to show that, biologically, caseinogen and lact-albumin 



