224 A MANUAL OF BACTERIOLOGY 



not always entirely specific, is usually very specialised ; 

 given proper precautions as to dilution, time-limit, 

 condition of test culture, etc., an anti -serum will generally 

 only agglutinate the homologous organism or closely 

 allied species that is, it is to some extent a group reac- 

 tion. If agglutination of allied organisms take place 

 " multiple, cross or co-agglutination," as it is termed it is 

 not so marked as that of the homologous organism. Anti- 

 typhoid serum, for example, may agglutinate not only 

 the typhoid bacillus, but also, though to a less degree, 

 members of the paratyphoid group. The degree of agglu- 

 tination is ascertained by determining the limit of dilu- 

 tion of the agglutinating serum which causes agglutina- 

 tion the homologous organism is agglutinated by a 

 higher dilution than an allied organism. As the result of 

 infection, agglutinins may be present in the serum which 

 similarly agglutinate not only the organism of the infec- 

 tion, but also other organisms e.g. typhoid serum may 

 agglutinate B. paratyphosus and J5. coli as well as the 

 B. typhosvis, and typhus serum x agglutinates B. typhosus 

 and M. melitensis ; the agglutination is, however, always 

 much more marked with the homologous organism. 

 The agglutinins acting on the infecting organism may be 

 termed primary or homologous, those acting on other 

 organisms secondary or heterologous. In a case of double 

 infection each organism may produce its own primary 

 agglutinin, so that the agglutination of two species by a 

 serum may be due to the presence either of primary and 

 secondary agglutinins or of two primary agglutinins. 

 Castellani, 2 by applying the saturation test, found that 

 an organism absorbs both its primary and secondary 



1 Typhus serum is peculiar in that it has considerable power of agglutinat- 

 ing entirely heterologous organisms. 



2 Zeitschr. f. Hyg., xl, 1902, p. 1. See Taylor, Journ. of Hyg., vol. xvii, 

 1918, p. 415 (Resume). 



