150 LIFE: ITS NATURE AND ORIGIN 



J. R. Marrack 22 gave an excellent review of both the chemical 

 and the physical aspects of immune reactions and pointed out the 

 fact that it is possible to distinguish two stages in immunity 

 reactions: (1) specific combination of "determinant" groups with 

 antibody, and (2) secondary reactions — precipitation, agglutina- 

 tion, etc. Many "determinative" groups lack chemically reactive 

 areas, and immunological equivalence may be shown by groups 

 chemically different, e.g., 



CH 3 



and 

 NH 2 NH 2 



Marrack states: "Such a result can only be due to intermolecular 

 forces and the specific character must be ascribed to (1) an 

 appropriate distribution of polar fields on the determinant group 

 and on the antibody; and (2) to purely spatial considerations, 

 since the approach of a determinative group to a receptive site on 

 the antibody may be prevented by an inert substituent which gets 

 in the way (steric hindrance). These are the same factors as deter- 

 mine the specific selection of the molecules which are built into a 

 crystal." 



It may here be noted that if adsorbed molecules are polar, which 

 is commonly the case, the pattern of the molecular "ends" ad- 

 sorbed on the oppositely charged areas of a complex surface 

 would tend to form a monolayer or plaque having the reverse 

 pattern of the surface on the near side and a duplicate pattern 

 of the surface on the off side. 23 Over 20 years ago Nellensteyn 

 found that diamond will adsorb methylene blue but not succinic 

 acid; but the reverse is the case with graphite, which differs from 

 diamond only in the spacing of the carbon atoms. While the 

 formation of pure crystals of simple compounds with exclusion of 

 "foreign" molecules may take place by the selective and oriented 

 specific "adsorption" of new particles at the various growing 

 crystal surfaces, in the case of very large molecules or molecular 

 groups, like antigens and antibodies, a "spot welding" of molecules 

 at limited reactive areas may suffice to form aggregates strong 

 enough and large enough to settle out. Since time is a factor here, 

 the drop in kinetic activity which accompanies particle growth 

 will, up to the zone of optimum colloidality, favor the proper 

 adjustment of attractive areas to each other. Moderate agitation 



