46 IMMUNITY 



\^ Alexins. It has been found by numerous observers, that normal 

 blood serum is germicidal for many bacteria, and the peculiarly 

 active substance that is contained in the serum, was called by Buch- 

 ner Alexin. This dissolves bacteria and destroys them. It also 

 destroys the red blood cells of other animals. The alexin of a dog's 

 serum dissolves the red cells of a rabbit; it is therefore hamolytic. 

 It also is thermolabile, that is, its properties are destroyed by heat 

 (55 C.). It is identical with the complement of Ehrlich, and the 

 cytase of MetchnikofL X^ 



><^The complement, as it will hereafter be called, takes, as already 

 stated, an active part in bacteriolysis, or bacteria-dissolving, and 

 in haemolysis, or blood-dissolving, it is present in normal non-im- 

 mune sera. R. Pfeiffer found that if some serum from a guinea pig 

 immunized against cholera spirilla is injected into the peritoneal 

 cavity of a healthy non-immune guinea pig, with some cholera 

 spirilla, that the latter are agglutinated, and ultimately^ dissolved, 

 having undergone bacteriolysis (Pfeiffer' s reaction) yj^The immune 

 serum alone in a test-tube, with the cholera spirilla does not have 

 this action, but if some normal guinea pig serum is added to the 

 mixture, an immediate solution takes place, showing that the 

 presence of both the normal serum containing the complement, 

 and the immune serum, containing the immune body, or am- 

 boceptor, are necessary to complete the solution of the bacteria. 

 If the complement is heated above 55 C. for an hour, solution 

 does not take place, even if the immune serum is present, but, 

 after heating the mixture, it may be reactivated by adding some 

 fresh unheated complement. The complement is thermolabile i.e., 

 destroyed by heat/) 



V_Xhe immune serum is not affected by heat, and is therefore called 

 the/mostabileT} 

 \JThese various reactions may be expressed concretely thus: 



Bacteria + immune body = no solution. 

 Bacteria-!- complement = no solution. 



