38 IMMUNE SERA 



to the need of salts in the agglutination of bacteria 

 by sera. In the latter reaction the agglutinins 

 carry a positive, the bacteria a negative charge. 

 The resulting combination, therefore, does not 

 precipitate from the menstruum, supposedly because 

 there is still sufficient difference in the electric 

 potential. When salts are present the kations so 

 alter the electric conditions of the colloidal par- 

 ticles, i.e., of the agglutinin-bacterium combina- 

 tion, that their surface tension is increased. In 

 order to overcome this the particles get together, 

 presenting in a clump less surface tension than if 

 they remained as individual particles. 



Agglutinoids. Agglutinins which have lost their 

 agglutinophore group through the action of acids, 

 etc., but which still possess their haptophore group, 

 are called agglutinoids, just as toxins which have 

 lost their toxophore group are called toxoids. 

 Such agglutinoids, then, may still combine with 

 the blood cells or bacteria without being able, how- 

 ever, to produce any clumping or agglutination. 

 The nature of agglutinoids, however, is still very 

 obscure as is also the means by which they inhibit 

 agglutination. It has occasionally been observed, 

 for example, that agglutination is absent in con- 

 centrated serum, and present in dilute serum. 

 This zone, of no agglutination preceding that of 

 agglutination is often spoken of as the pro zone. 

 It has been explained as due to the presence in the 



