4l8 HUMAN BIOLOGY 



injury. We can, for instance, inject antibodies against 

 horse serum into a normal guinea pig and, allowing time for a 

 penetration of these reaction bodies into the cells, render the 

 an mal "passively' sensitive to horse serum. Quantitative 

 exper ments indicate that the power of an "antihorse serum" 

 thus to sensitize a normal animal "passively" is propor- 

 tionate to the contents of "antihorse" antibodies. And it is 

 a logical deduction, therefore, that, knowing the reactions 

 to be cellular, the cells have absorbed the specific antibodies, 

 now containing, in the jargon of our trade, "sessile receptors 

 or antibodies" for horse serum. For many reasons it is 

 clear that a similar mechanism underlies most of the other 

 forms of hypersensitiveness which are concerned in important 

 pathological conditions of man, namely, asthma, hay fever, 

 serum sickness and perhaps drug idiosyncrasies. And in 

 many of these conditions, although the fundamental phe- 

 nomena are identical with those encountered in hypersensi- 

 tiveness to horse serum and other proteins, circulating 

 antibodies have not been demonstrable. For many reasons 

 which it is quite impossible to analyze in this connection, 

 it seems probable that in several of these conditions the 

 antigen is quite capable of arousing a specific change in 

 the reaction capacity of the cells analogous in every way to 

 that aroused by the proteins but not followed by the pro- 

 duction of circulating antibodies. We would, accordingly, 

 define the term "antigen" today not as representing only 

 substances that lead to antibody formation, but as any 

 material which, introduced into the physiological interior of 

 the body, leads to a specific alteration of the reaction 

 capacity of the cells, detectable now not only by the presence 

 of antibodies, but by the development of local or general 

 hypersensitiveness to the particular substance. Nevertheless, 

 while it is, of course, important to recognize that there is a 

 difference between those foreign substances which give 

 rise to antibodies and those for which no circulating anti- 

 bodies appear, it is quite as necessary to remember that 

 the fundamental occurrences in all forms of hypersensitive- 

 ness are alike in indicating specific alterations in cell 

 responses, identical in all phases except in those which 

 depend upon the presence of the circulating antibodies. 



