162 INFECTION AND RESISTANCE 



bactericidal tests with any frequency is sure to meet with the phe- 

 nomenon again and again. But their explanation, which involves 

 the assumption of union between free sensitizer or amboceptor, and 

 alexin or complement, without the participation of antigen, cannot 

 be accepted since, search as we may, through the extensive experi- 

 mentation that this problem has inspired, there is no instance on 

 record in which indisputable evidence of such an occurrence has 

 been advanced. On the contrary, there is a mass of satisfactory 

 evidence available which indicates clearly that amboceptor or sensi- 

 tizer alone cannot absorb alexin, and the Neisser-Wechsberg explan- 

 ation seems consequently to be merely an interesting and cleverly 

 conceived but improbable possibility. 



What, then, is the explanation of the diminution of bactericidal 

 effect in the presence of an excess of sensitizer ? We will see that, in 

 the study of agglutinin and precipitin reactions, phenomena exactly 

 analogous to the Neisser-Wechsberg effect have been noticed, in the 

 case of the agglutinins, the so-called "pro-agglutinoid" zone being a 

 case in point. For these phenomena, as well as for that of 

 Neisser and Wechsberg, explanations have been advanced by 

 the Ehrlich school, similar in principle in that they all depend upon 

 more or less arbitrary assumptions regarding affinity between the 

 reacting bodies. Such explanations, though not outside the realm of 

 possibility, have, however, lost much force since it has been recog- 

 nized that the reactions between serum antibodies and their antigens, 

 in general, take place according to laws far more closely analogous 

 to those governing reactions between colloids than to those governing 

 chemical reactions in which the laws of definite proportions can be 

 applied. And, indeed, the reacting substances in antigen-antibody 

 complexes are, beyond doubt, of the nature of colloids. Now, in 

 many precipitations resulting when two colloids are mixed, an ex- 

 cess of one or the other factor will completely inhibit the occurrence 

 of the precipitation; the reaction taking place only when definite 

 proportions between the reacting bodies are present. The occurrence 

 of such inhibition zones, due to an excessive concentration of one 

 reagent, can be shown for agglutination and precipitation, exactly 

 as it can in ordinary colloidal reactions, and it is more than likely 

 that the Neisser-Wechsberg phenomenon is merely an example of a 

 similar phenomenon. 



Looked at from this point of view, far from supporting the sup- 

 position of a separate complementophile group and therefore of the 

 "amboceptor" nature of the heat-stable lytic antibody, the Neisser- 

 Wechsberg phenomenon indeed becomes rather a strong argument 

 in favor of Bordet's views, and against those of Ehrlich. For, by 

 introducing the analogy between the lytic and bactericidal processes 

 with colloidal reactions, it takes away much force from the supposi- 

 tion that antigen-sensitizer alexin reactions take place according to 



