THE MICROSCOPIC APPEARANCE OF SPECIFIC AGGREGATES 223 



(Hershey 1944). The resolution of the controversy either by the establishment 

 of one or the other hypothesis, or their merging into a more general hypothesis, 

 must await further research. 

 The Microscopic Appearance of Specific Aggregates. 



There are few observations of specific aggregates, either during or after their formation, 

 which yield any information about the nature of the antigen-antibody union. Pijper 

 (1938, 1941a, b) records an interesting study of the agglutination of Salm. typhi made 

 by dark-ground cinematography. He confirms the well-established fact that flagellar 

 antibody first immobilizes the organisms by a du'ect action on the flagella, after which 

 aggregates are formed by the adhesion to one another of the motionless flagella of bacteria 

 fortuitously brought into contact by Brownian movement or convection currents (Fig. 37). 

 Agglutination by antibody to the somatic antigens results in the formation of closely 

 packed clumps of bacteria, which retain their motility since the flagella are not immobilized. 

 The cinematograph record shows the motile, sensitized bacteria swimming in aU directions 

 across the field along relatively straight fines ; and many of them are seen swimming 

 directly towards already formed aggregates, a phenomenon that Pijper attributes to 



Fig. 37. — Dark-ground photomicrograph of Salm. typhi aggregated by flagellar antibodies. 

 (From a photograph kindly supplied by Dr. A. Pijper.) 



attraction exerted by the aggregate. It is diflficult to imagine that the umon of antigen 

 and antibody could generate a field of force capable of acting at distances of several 

 microns, and it appears to us that the apparent attraction can be explained by postulating 

 a straight-fine course for the motile sensitized bacilli over short distances. If an aggregate 

 is in the fine of travel, a head-on coUision with the aggregate, and adhesion to it, will give 

 the impression of attraction. 



The bacterial aggregates formed during agglutination varied with the antibody used. 

 Sensitization by flagellar antibody gives the fortuitous pattern already noted ; by somatic 

 O antibody, an end-to-end arrangement (Fig. 38) ; and by Vi antibody (see Chapter 8), 

 the characteristic packing depicted in Fig. 39. The factors that determine these patterns 

 are matters for speculation. It may be noted in this connection that Miles and Pirie 

 (1939) observed specific precipitates of antibody and native antigen from Br. melitensis 

 which appeared to be built up of bacterial fragments adhering to each other end-to-end. 



During the action of flagellar antibodies, Pijper observed the deposition of granules 

 on the flagella. Mudd and Anderson (1941) found a patchy distribution of thickenings 

 on the flagella of specificaUy sensitized Salm. typhi. Measured in electron micrographs, 

 the increase in thickness varied from 1 to 17 m/x. Similar thickenings occurred on the 

 cell waU. The dimensions of the thickenings were of an order of size compatible with 

 those of a unimolecular layer of rabbit antibody. 



