174 STUDIES IN IMMUNITY. 



learned that the red blood cells of an animal are not attacked by the 

 alexin from the same animal, whereas they are susceptible to alexins 

 from other animals. 



We might compare in a very gross way the modification which 

 the sensitizing substance causes in the corpuscles by likening it 

 to a change in the structure of a lock by means of which one or 

 several keys that previously could not open it are enabled to do so. 

 Any two keys that are sufficiently alike would enter the lock in- 

 differently. In the same way the alexins, both of the guinea-pig 

 and of the rabbit, "enter" rabbit corpuscles or hen corpuscles 

 once these corpuscles have been sensitized. On the other hand, 

 hen alexin, that apparently differs too greatly from guinea-pig or 

 rabbit alexin, does not " enter" hen corpuscles even when they are 

 previously sensitized. It may be that the entrance or the efficient 

 working of the alexin depends, not only on the nature of the alexin, 

 but also on the nature or the particular origin of the sensitizing sub- 

 stance. There are evidently many experiments yet to be made. 

 In the absence of sufficient data we shall content ourselves for the 

 moment with pointing out a method of obtaining some insight into 

 the mode of action of these substances. 



The effect of heat on the active substances of serum. On heating 

 rabbit serum, active against hen corpuscles, to 55 degrees the agglu- 

 tinating and sensitizing properties persist, whereas, as we have 

 already seen, the dissolving property is suppressed. 



On heating for half an hour to 65 degrees, both the agglutinating 

 and the sensitizing power are still intact; heating to 65 degrees 

 diminishes the agglutinating property slightly, and it becomes very 

 weak on heating to 70 degrees for the same length of time. It should 

 be remarked at this point that unheated serum when previously 

 diluted with, say, 20 parts of salt solution has no visible agglutina- 

 ting effect in any dose, which shows that the agglutinin must be 

 present in a certain state of concentration in order to affect cor- 

 puscles, and, moreover, when the phenomenon is absent no conclusion 

 can be drawn as to complete absence of the active substance. The 

 sensitizing substance, however, is still distinctly manifest in serum 

 heated to 70 degrees and also in serum that has been diluted with 

 20 parts of salt solution. One part of defibrinated blood mixed 

 with two parts of serum heated to 70 degrees plus four parts of 



