200 IMMUNOLOGY 



antibody union. The one theory first suggested by Bordet and sup- 

 ported by experimental data outlined in Chapter XII assumes that 

 the agglutination or precipitation is due to the reduction in re- 

 pelling force between the cells or colloidal particles and the ex- 

 istence of cohesive properties of the sensitized antigen. Accord- 

 ing to this theory these physical changes in state of aggregation are 

 secondary phenomena in which chemical forces do not play a role. 

 The second theory, supported by experimental work of Heidel- 

 berger and Kabat (1937), statas that chemical forces play a role 

 in the second stage wliich is agglutination of cells or precipitation 

 of colloidal particles. Opposition to this latter concept is offered 

 by Hooker and Boyd (1938) and Eagle (1938). The explanation 

 we offer in Chapter XII is largely in harmony with the explana- 

 tion offered by Bordet, Eagle, Hooker and Boyd and others. 

 We realize there is considerable evidence in support of the op- 

 posing view but until there is a general acceptance of it we will 

 hold to the older theory. 



Method of Measuring- Antibodies. — 



Antitoxins. — Ehrlich suggested a method of standardizing 

 antitoxin, which, while used to some extent, is being superseded 

 by the Ramon flocculation technique. Ehrlich was working with 

 diplitheria toxin and determined the smallest amount tliat would 

 kill a 250 gi-am guinea pig in four or five days. This he called a 

 minimum lethal dose or an M.L.D. He then measured out 100 

 M.L.D.'s and called that amount an Lq dose of toxin. Tlie amount 

 of antitoxin that just neutralized tliis he called a luiit of anti- 

 toxin. This amount corresponds to what is now called an " old 

 unit." He then determined the least amount of toxin tluit, when 

 mixed with an ''old" unit of antitoxin, would kill a 250 gram 

 guinea pig in four to five days. This amount of toxin he called the 

 L, dose. The least amount of antitoxin which protects the guinea 

 pig against the L^ dase of toxin is the "new unit." In Chapter 

 XIV the method of titrating antitoxin by the Ramon flocculation 

 technique is given. Eaton (1936) discusses the flocculation re- 

 action with purified diphtheria toxin. This method is also ap- 

 plied to standardizing tetanus and other antitoxins. 



Unit of Tetanus Antitoxin.— The American immunity unit 

 of tetanus antitoxin has been defined as ''ten times the least 



