OTHER MANIFESTATIONS OF ANTIGEN-ANTIBODY REACTION 241 



state (see Zinsser 1930), devised a method of detecting small non-precipitating amounts 

 of antibody by titrating them against suspensions of collodion particles coated with the 

 appropriate antigen (see also Lowell 1943). 



The fine turbidity which precedes flocculation in strongly reacting antigen-antibody 

 systems, and which may be the end-result in weakly reacting systems, can be measured 

 by photoelectric and other types of nephelometer, and the measures made the basis of 

 titrations of antigen or antibody (see, for example, Libby 1938, Pope and Healey 1938, 

 Bukantz, Cooper knd BuUowa 1941). Advantage may also be taken of physicochemical 

 properties of antigen-antibody compounds other than precipitabUity in the presence of 

 electrolyte. Thus, du Noiiy and Hamon (1935, 1936) found a weU-marked maximum of 

 viscosity to occur in mixtures containing optimal proportions of diphtheria toxin and 

 antitoxin. 



After initial sensitization, which is relatively rapid, the agglutination of bacteria 

 depends on their rate of collision under the influence of Brownian movement and con- 

 vection currents in the suspending fluid. If the cells are brought together rapidly by 

 centrifugation, agglutination may be rapidly detected by the resistance to resuspension 

 of the centrifuged mass, as compared with that of a control preparation of bacteria without 

 antibody. 



An interesting consequence of an antigen-antibody reaction is the swelling of pneumo- 

 coccal capsules in the presence of specific anti-capsular rabbit serum, first described by 

 Neufeld 1902, Neufeld and Etinger-Tulezynska 1931 (see Chapter 24). The swelling 

 reaction can be elicited in both living cajjsulated pneumococci, and in desiccated organisms 

 (Brown 1938). The swelling may be reversed if the antibody is destroyed by heat (Etipger- 

 Tulczynska 1933) or by digestion of serum proteins with papain (Kalmanson and Bronfen- 

 brenner 1942). It is reversed if the antibody is removed by repeated washing, or by 

 adding an excess of specific polysaccharide to a suspension of washed swollen cells (Kempf 

 and Nungester 1942). 



According to Hershey (1940), the capsule does not appear to present a smooth surface 

 to the action of antibody, for the amount absorbed is greater than such a surface could 

 accommodate as a closely packed unimolecular layer ; it is possible that the capsular 

 polysaccharide is in the form of a loose fibrous gel, whose surface and interstices provide 

 ample surface for the adsorption of large amounts of antibody. The electron-micrographic 

 studies of Mudd and his colleagues support this view. As we have already noted, homo- 

 logous antibody is deposited on Salrn. typhi in an electron-opaque layer about as thick 

 as a unimolecular film of globuhn (Mudd and Anderson 1941). Under the influence 

 of rabbit antibody, the pneumococcal capsule, which is relatively transparent to the 

 electron beam and appears to be a low density gel, becomes opaque, and increases in 

 thickness. The increase in thickness is more than 25 times that of a unimolecular layer 

 of antibody, even assuming that the globuhn is packed with its maximum length per- 

 pendicular to the surface of the capsule (Mudd, Heinmets and Anderson 1943). There 

 is clearly some adsorption of non-specific material, for the opacity of swollen capsule is 

 increased in the presence of non-specific serum proteins. It seems probable that the 

 sweUing is not purely an antigen-antibody effect. Johnson and Deimison (1944) measured 

 the volume increase during swelling by two independent methods ; it varied from 2-9 

 to 9-5 /Li^ by one method, and from 7-4 to 10- 1 /n^ by the other. They concluded that 

 the volume increase could not be accounted for solely by the molecular volume of anti- 

 body absorbed, but might be due to a hydration of the capsule following the antigen- 

 antibody reaction. 



We may refer to the " adhesion phenomenon," in which spirochaetes specifically 

 sensitized by antibody adhere readily to bacterial cells added to the reacting mixture; 

 Advantage is taken of this behaviour in the serological study of spirochaetes (p. 912). 



