208 THE ANTIOEN-ANTIBODT REACTIONS 



extracted rabbit, rat, guinea-pig and sheep antisera, and lecithin that of extracted horse, 

 man. mouse, cat, dog and goat antisera. Although it is possible that a small amount 

 of Upin is an essential constituent of antibody globulin (('how and Goebel 1935), the large 

 amounts often found in specific precipitates are adsorbed non-specifically during precipita- 

 tion, and are essential neither to the complete antibody molecule nor to precipitation. 

 Native antibody may be a lipo-globuUn complex containing this small amount of lipin 

 (see Chow and Goebel 1935). Heidelberger (1939) suggests that the action of lipin is 

 essentially mechanical, providing nuclei round which specific precipitation is initiated. 



The lubibiting Effect on Precipitation of Antigen or Antibody Excess. 



We may now return to the inhibitory effect exerted by antigen or anti- 

 body excess, and to the observed difference between the constant-antibody O.R. 

 and the constant-antigen O.R. 



Boyd and Hooker (1934, 1938) pointed out that the ratio of antibody to antigen 

 in optimal precipitates varied inversely as the molecular weight of the antigen. 

 For example with the Type III pneumococcal polysaccharide, with an estimated 

 molecular weight of 4,000, the mean ratio was 60 ; with serum albumin of mole- 

 cular weight 70,200, the mean ratio was 7-4, and with a mollusc hsemocyanin of mole- 

 cular weight 3,000,000, the mean ratio was 1-53. The relation holds with sufficient 

 accuracy over a whole series of precipitin systems. Since the molecular weight 

 of the rabbit and horse antibodies used in the majority of the systems was in the 

 region of 150,000, it follows from the size of the ratio that the antigen molecules 

 concerned must each possess a number of similar reactive groups capable of com- 

 bining with antibody. At the optimum, the amount of antibody taken up is 

 consistent with the hypothesis that it forms a layer of contiguous units of antibody 

 protein (see also Kleczkowski 1941a). 



We may call the reactive groupings on antigen and antibody " valencies," 

 though, unlike the valency of orthodox chemistry, they are not simple atomic 

 valencies, but combining complexes. On this basis, antigens are multivalent, a 

 fact which may also be deduced from the reaction of antigens with artificially 

 introduced reactive groups. (See p. 262). 



There is more doubt about the valency of antibody. Analysis of precipitates 

 formed in the presence of a great excess of antigen should help to decide this point 

 by telling us the maximum number of antigen molecules that combine with a single 

 molecule of antibody (see Hooker and Boyd 1942). Unfortunately, for technical 

 reasons, the composition of precipitates in this region is difficult to determine 

 with accuracy, but the available figures indicate that usually one, and, in rare 

 cases, two, antibody molecules combine with one of antigen. On this basis, there- 

 fore, antibody is usually monovalent, or at most divalent. 



The " Two-Stage " and the " Lattice " Hypotheses. 



If we accept the hypothesis of the monovalent antibody, we are committed 

 to an explanation of the phenomena of precipitation along the Lines of Bordet, 

 namely that there is first a stage of specific combination, followed by a second 

 stage of non-specific flocculation depending on the presence of electrolytes and 

 serum lipins. The union of antibody globulin with antigen renders it hydrophobe, 

 and consequently salt-sensitive. The antigen-antibody complexes are forced out 

 of solution, and having more attraction for one another than each separate particle 

 has for water, they cohere and eventually form a visible precipitate. This is the 

 " two-stage " hypothesis. 



On the other hand, there are phenomena in precipitation and agglutination 



