ZONES OF INHIBITION 225 



agglutinins, but does not absorb two primary agglutinins. 

 This test would, therefore, distinguish a double infection 

 from a single one. Thus, if a typhoid serum agglutinated 

 both the B. typhosus and the B. coli, and the serum after 

 saturation with typhoid culture still agglutinated the 

 B. coli, this would point to an infection with the latter as 

 well as with B. typhosus. The formation of primary and 

 secondary agglutinins may be brought about as follows : 

 The bacterial cell may be regarded as consisting of a 

 complex of substances, all of which act as antigens and 

 form agglutinins the agglutinin produced by the cell is 

 really a mixture of closely allied agglutinins. In the case 

 of the cells of two bacterial species, the sera formed by 

 which exhibit cross-agglutination, it may be conceived 

 that a small portion of the cell complex of each species 

 is identical, or almost so, and will therefore form some 

 common agglutinin the secondary agglutinin -and, con- 

 sequently, the serum produced by each bacterium tends 

 to agglutinate the other organism. 



A remarkable phenomenon observed in connection with 

 agglutination in the case of natural infections, which the 

 writer has particularly noticed in undulant fever, is the 

 occurrence of what may be termed a zone of no reaction 

 or of inhibition with some particular dilution of the 

 serum. Thus, dilutions of 1 in 20 and 1 in 30 may agglu- 

 tinate strongly ; a 1 in 40, however, may hardly agglu- 

 tinate at all, while dilutions of 1 in 50 and upwards to 

 1 in 100 or more may agglutinate well. A similar phe- 

 nomenon is also observed with prepared agglutinating 

 sera. 



Agglutination is not confined to micro-organisms, but 

 other cells may give rise to agglutinins which agglutinate 

 them. Thus, anti-serum, prepared by injecting erythro- 

 cytes, agglutinates the red blood-corpuscles, and in certain 

 diseases, e.g. pneumonia, chromocyte clumping may be a 



M.B. 15 



