IMMUNITY. 227 



there are two bodies concerned: both are agreed as to the property of the one to 

 be readily destroyed by heat, of the other to resist heat. 



In English writing it is more common to use the German than the French 

 te ms, so these will be employed in the present case, though a great deal of what 

 is known about lysins has been contributed by Bordet and the French school 

 generally. The word alexin was first used by Buchner, but is used now mostly 

 in French writings. 



It should be recalled that a lysin is the substance formed in the blood-serum 

 of an animal when the latter is injected with bacteria or with foreign red blootl 

 cells. A rabbit injected with typhoid bacilli develops lysin for typhoid bacilli; 

 when injected with red blood-cells of a guinea-pig, develops a lysin for guinea- 

 pig red cells. Lysins are not only produced artificially by such injections bui 

 they may also be present in blood-serum normally. 



If the lysin is heated to 55 C. for thirty minutes, it loses its complement (or 

 alexin} and the immune body (substance sensibilisatrice) only remains; so that 

 red cells which are disintegrated by the unhoated lysin remain intact in the heated 

 lysin. But the heated lysin becomes active again if either fresh unheated rabbit's 

 or guinea-pig's serum is added to it. The heated lysin is spoken of as inactivated- 

 the heated lysin with fresh serum, reactivated. The fresh serum which is added 

 contains the complement (alexin); the heated lysin contains only the immune 

 body (substance sensibilisatrice). 



The immune body is specific, but the complement is not; at least the blood of 

 Some animals contains complements for several different immune bodies. Thus 

 fresh horse serum added to various inactivated lysins reactivates the latter. 

 But chicken blood-serum does not contain complement for chicken corpuscles. 

 For if chicken hemolysin, produced by injecting a rabbit with chicken red cells, 

 is heated to 55 C. for thirty minutes (inactivated), it will not disintegrate chicken 

 red cells if fresh chicken serum be added; but if fresh rabbit serum is added, 

 it will hemolize chicken red cells as it did before heating. 



The immune body becomes fixed to the red cells, as can be shown by adding 

 red cells to inactivated lysin and then washing these with salt solution. If after 

 adding the red cells to the inactivated lysin the mixture is centrifugalized and 

 the precipitated red cells washed with salt solution so as to remove all of the free 

 immune body, the precipitated, washed red cells disintegrate when fresh comple- 

 ment i. e., fresh serum is added. 



From these and other considerations Bordet regards lysin as composed of a 

 specific antibody, sensibilisatrice or immune body of Ehrlich, on the one hand 

 and of a cytolytic, bacteriolytic, hemolytic alexin proper or complement of Ehr- 

 lich, on the other. The immune body is specific, but it does not cause destruc- 

 tion of cells by itself: it does so only in conjunction with complement or alexin. 

 Alexin is not strictly specific, and it has some cytolytic power, as seen in normal 

 blood independently of substance sensibilisatrice. But its power is greatly en- 

 hanced if the cells acted upon are first sensitized by sensibilisatrice. 



The reactions found by Bordet may be briefly summarized as follows: 



