A GENERAL RESUME OF IMMUNITY. 525 



tained as Landsteiner, Pick and other scientists have done by 

 extracting corpuscles or bacteria in colloidal suspension, are inter- 

 esting from the standpoint of the selective action of serum. Slees- 

 wijk and I have recently studied the action of hen serum on the 

 lipoids extracted from red blood cells by methyl alcohol and 

 subsequently made into the form of an emulsion in salt solution. 

 We found, for example, that hen serum agglutinates the lipoids 

 extracted from rabbit corpuscles energetically, but has very little 

 effect on those from bovine corpuscles. The agglutination of 

 an emulsion of lipoids is certainly an adsorption phenomenon. In 

 this case adsorption affinity, of varying intensity in accordance 

 with the substances employed, may certainly be stated as the cause 

 of the differences observed when a given serum is mixed with differ- 

 ent bacteria or corpuscles. And, moreover, the adhesion properties 

 are a function not only depending on the chemical constitution but 

 also on the physical state of the substances. It is easily conceivable, 

 then, that the aptitude of the antigen to react with the antibody may 

 disappear when affected by slight alterations or simple physical 

 modifications. It is easy to understand, then, why, in Obermeyer 

 and Pick's experiments, the serum obtained by immunizing animals 

 against certain albuminous substances differed from the serum 

 obtained by immunizing animals against the same substances pre- 

 viously heated. In brief, although an explanation of the specificity 

 of sera is not yet clearly evident, we have at least the right to con- 

 ceive of it as harmonious with the conception of adsorption affinity 

 as the essential factor. 



* * 

 This important problem of specificity must appeal to all bacteri- 

 ologists and I have recently begun to study it experimentally 

 with my collaborator, Sleeswijk. Inasmuch as our researches are 

 not quite finished, I may content myself with mentioning certain 

 facts. The problem of specificity may, obviously, be regarded from 

 two different aspects. When, for example, we study, an antibacterial 

 serum we wonder, on the one hand, why the serum is specific, and on 

 the other hand, why the organism allows itself to be specifically 

 affected. In other words, both the antibody and the affected ele- 

 ment must be considered. I cannot deal here with the second half 

 of the question. 



