THE PHENOMENON OF PRECIPITATION 271 



lead to a sudden elimination of the colloidal protective action, union 

 of the antigen and antibody, and, by the mechanism just outlined, 

 anaphylactic shock. 



The fact, moreover, that mere heating will change the precipi- 

 tating action, which certain sera have on inorganic colloids, to a 

 protective one seems to show that this latter function may justly 

 be associated with delicate physical or chemical alterations of animal 

 sera. 



Furthermore, this point of view is strengthened by the fact that 

 the mutual precipitation of sera, such as those described, takes place 

 slowly, as does the mutual precipitation of two protected colloidal 

 mixtures, in contradistinction to the more rapid precipitation which 

 takes place when any of these sera is added to an antigen dilution, 

 where the element of protection may be assumed to be practically 

 eliminated by more extensively changed quantitative relations. 



