IMMUNITY. 191 



The theories of Ehrlich and Bordet in regard to the composition of lysins 

 may also be appropriately discussed in this connection, as it is from the studies 

 of these bodies that many of the ideas in regard to immunity have been devel- 

 oped. 



A lysin contains two substances, a, thermolabile and a thermostabile sub- 

 stance — i. c, one readily destroyed by heating at 55° C. for a half-hour, the 

 other resisting much higher temperatures. The thermostabile substance is 

 now called by Ehrlich the immune body, the thermolabile the complement, 

 though Ehrlich has used in the past various other names for these hypothetical 

 bodies. Bordet uses the name substance sensibilisatrice for the thermostabile, 

 Ehrlich's immune body, and alexin for the thermolabile or Ehrlich's com- 

 plement. Both are agreed that 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 EngHsh writing it is more common to use the German than the French 

 terms, so these will be employed, 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 writ- 

 ings. 



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 blood- 

 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. 



If 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 unheated lysin remain intact in the 

 heated lysin. But the heated lysin becomes active again if either fresh un- 

 heated 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 cor- 

 puscles. 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 disinte- 

 grate chicken red cells if fresh chicken serum be added; but if fresh rabbit 

 serum is added, it will hemoUze 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 



