2 PROPERTIES OF ILEMOLYTIC SERA 



the antigen, filling up as it were its combining affinity, 

 but without producing any other physical change so far 

 as is known. The anti-toxins are the typical members 

 of this class. In the second class, the anti-substance com- 

 bines with the antigen and also, by a zymotoxic action, 

 produces some recognizable physical change in it, resulting, 

 for example, in precipitation, agglutination, &c. In the 

 third class the anti-substance combines with the antigen 

 without of itself bringing about any recognizable effect 

 in it so far as we know, but it leads to the union of the 

 labile substance in normal serum, generally known as 

 ' complement ' or alexine, which often produces some 

 destructive action. The bacteriolytic and heemolytic sera 

 are the typical examples. It was formerly supposed that 

 anti-substances of the third order of this nature were only 

 produced when formed elements were used for injection, 

 but it is now known that they can also be developed by 

 apparently simple protein substances in solution. From 

 the biochemical point of view the distinctive action of an 

 anti-substance of this order is its leading to the combination 

 of complement. 



In the case of a hsemolytic serum we have, therefore, three 

 kinds of molecules concerned: (1) the receptors (R) of the 

 red corpuscles which act as antigens : (2) the corresponding 

 anti-substances developed by immunization and possess- 

 ing the specific affinity for the antigens, they are usually 

 known as immune-bodies (IB) ; and (3) the normal con- 

 stituent of the serum, complement (C), which unites through 

 the medium of the immune-body ; it does not increase 

 during the process of immunization. The lysis of the 

 corpuscles by the heemolytic serum may thus be said to 

 result from the union of R 4- IB + C. It follows that given 

 any two of these substances, we can test for the presence 

 of the third. This will be abundantly illustrated by the ex- 

 periments detailed. The relations of the several substances 



