200 THE ANTIOEN-ANTIBODY REACTIONS 



not analogous to that between a strong acid and a strong base, but is a reversible 

 action of the type which occurs between weak acids and weak bases, the equilibrium 

 attained in any particular case being determined by the concentration of the reacting 

 substances, in accordance with the law of mass action. 



Our present concept of the mechanism of the antigen-antibody reactions is 

 derived, in varying degree, from each of these three theories. Its essential core is 

 provided by Ehrlich's classical hypothesis, simplified and made more precise by 

 the pioneer chemical studies of Landsteiner and his colleagues (see pp. 252-258). 

 It follows Bordet in postulating the union of antigen with antibody in varying 

 proportions. It adopts the assumption of Arrhenius and Madsen that the antigen- 

 antibody compound is dissociable, at least in some cases and under certain 

 conditions. 



Before discussing how this synthesis of conflicting theories has been brought 

 about, it will be convenient to give a brief description of each of the antigen-antibody 

 reactions in turn. 



The Precipitin Reaction. 



Our knowledge of the mechanism of this reaction has, in the main, been obtained 

 during recent years ; but it forms so convenient a starting-point for a general descrip- 

 tion of the in vitro reactions between antigens and antibodies that we may with 

 advantage ignore historical succession. In this reaction the antigen, as well as the 

 antibody, is initially in a state of colloidal solution. We can therefore work with 

 chemically purified antigens, such as crystalline egg albumin, or one of the bact- 

 erial polysaccharides that will be described in future chapters. In this way we can 

 eliminate the complexities that are introduced by the presence in the reacting system 

 of several different antigenic components. We can also, as Landsteiner and his 

 colleagues have shown (see pp. 252-255), prepare synthetic antigens, the specificity 

 of which is determined by known chemical groupings, and study their flocculation 

 by antisera prepared against them. Moreover, in the precipitin reaction our data 

 in regard to the quantitative relationships in our reacting system have attained 

 their greatest precision. 



Specific, serological precipitation occurs when any antigen in solution is allowed 

 to react with its corresponding antiserum in the presence of electrolytes, provided 

 that the concentration of each of these reagents, and the experimental conditions, 

 such as temperature of incubation and time of incubation, are suitably arranged. 



The rate of formation of the antigen-antibody compound, and of its flocculation, 

 varies according to the conditions of the experiment. It is hastened by any pro- 

 cedure that increases the frequency of impact between the molecules, or particles, 

 of antigen and antibody, or between the first-formed particles of the antigen- 

 antibody complex (see Eagle 1932). Such procedures include any decrease in the 

 total volume of fluid in the reacting system, with a consequent decrease in the space 

 between the reacting particles, and any factor that increases their rate of move- 

 ment, such as shaking or convection currents in a tube partially immersed in a water- 

 bath. It is also hastened by increasing the concentration of electrolytes up to 

 an optimal point, beyond which a further increase may cause a retardation. An 

 increase in temperature up to an optimum also increases the speed of flocculation, 

 and it seems doubtful whether this effect is wholly due to an increased frequency 

 of impact. With some reacting systems the higher water-bath temperatures 

 (37-55° C.) may be found unsuitable, since the antigen-antibody compound may be 

 partly soluble in this range. 



