24 CONSTITUENTS OF MILK 



"^Reductases. The ferments which cause the abstraction of 

 oxygen from compounds without the production of gaseous 

 oxygen, have been termed reductases. The essential differ- 

 ence between this reaction and that produced by catalases is 

 in the utiUsation or transference of the oxygen removed. 



Two tj^es of reductase have been recognized and are dif- 

 ferentiated by their action on methylene blue. One type, 

 which appears to be of cellular origin and is present in fresh 

 milk, rapidly decolourises methylene blue solutions in the 

 presence of a trace of formaldehyde, whilst the other is capable 

 of effecting the reduction in the absence of formaldehyde and 

 is of bacterial origin. 



Biological 



Immune Bodies. Although the examination of milk for the 

 presence of immune bodies is but infrequently required in con- 

 nection with pubUc health work, a general consideration of 

 these bodies and their significance is of interest. Before con- 

 sidering these in detail it will be advisable to review briefly the 

 theory of immunity. 



After an attack of disease-producing organisms, animals 

 usually possess, for a varying length of time, an immimity 

 against a further attack, and this immunity is ascribed to the 

 presence of substances known as immune bodies. The re- 

 searches of Ehrlich and others have established that these 

 immune bodies, or anti-bodies as they are generally described, 

 are produced by external agencies. In addition to living and 

 dead bacteria, other substances such as animal and vegetable 

 proteins, animal cells, and toxins, may act as antigens. Ehr- 

 lich's theory of immunity hypothecates the existence, in the 

 molecules constituting both the antigen and body cell, of 

 binding groups or haptophoric receptors which fit " as a key 

 fits the lock " and which anchor the antigen to the body cell. 

 In the case of toxins, other receptors are also assumed to be 

 present, viz., toxophores, which are responsible for the toxic 

 effects produced after the antigen has been anchored to the cell. 



