194 MICROBES AND TOXINS 



Complement or Alexine. — Before the discovery of the 

 antibodies, at the time when attempts were being made to 

 transfuse human patients with the blood of other mammals, in 

 particular of the sheep, it had been noticed that the normal 

 blood-serum of certain animals destroys the red corpuscles of 

 other species : Buchner attributed this property to a defensive 

 substance which he called alexine. Its chemical composition 

 is unknown ; it is thought to be an albuminoid substance : it is 

 known to disappear from serum on dialysis and to act after a 

 period of inactivity or incubation ; it is destroyed by tempera- 

 tures about 56° C, and only acts in presence of salts. Buchner 

 classes it along with the digestive ferments. 



The serum of many animal species, expressed frorn the blood 

 after coagulation, possesses the property of killing in vitro many 

 infective bacteria without any apparent assistance from the 

 body-cells : this destructive action is similarly attributed to the 

 alexine or complement, and it is this bactericidal property 

 which represents the simplest and crudest fact on which the 

 whole structure of the humoral theory of immunity has been 

 reared. 



The Two Substances Bordet showed in 1895 that the 



serum of an immunized animal contains two substances, or 

 rather two functions. Take a guinea-pig which has received 

 inlraperitoneally spaced injections of cholera vibrios : its serum 

 now destroys these in vitro, and is said to be bacteriolytic. 

 Heated to 56° C, it loses this property, but on the addition of 

 a little fresh serum, recovers it. The bacteriolytic action thus 

 demands primarily a substance present in the fresh normal 

 serum of any animal species (non-immunized), a substance 

 which heating to 56° C. destroys : it is the alexine or comple- 

 ment. But it demands also another substance present in the 

 serum of treated animals and absent in normal serum, which is 

 not destroyed by heating under 65° C. 



Bacteriolysis (the destruction of bacteria by serum) is thus 

 due to the collaboration of two functions, by custom regarded 

 as two substances. One, thermolabile, is the alexine or 

 complement and exists in all freshly prepared normal sera. 



