120 IMMUNITY AND INFECTION 
nity observed in the dog appears to be due to phagocytic activity of 
leukocytes which engulf and destroy anthrax bacilli which may have 
gained entrance' to the body.^ The rat, on the contrary, enjoys 
immunity not because its leukocytes engulf and destroy anthrax 
bacilli, but because the blood of the rat possesses soluble, non-specific 
bactericidal substances which destroy anthrax bacilli. Frequently 
both the cellular and humoral elements are engaged either simultane- 
ously or successively as the struggle between host and in^'ading organ- 
ism proceeds. 
Q 
K>'. 
Fig. 5.— Side-chains, first order (antitoxins and antiferments). 1, side-chain attached 
to cell; c, haptophore group; 2, side-chain to which is attached a toxin molecule; 3, 
a cast-off side-chain of the first order: antitoxin or antiferment; 4, a toxin or enzyme 
molecule; a, toxophore group; b, haptophore group; 5, a toxoid: the toxophore group 
is destroyed, leaving the haptophore group (b) intact; 6, a toxin molecule attached to 
a cast-off side-chain (antitoxin), illustrating the neutralization of toxin by antitoxin in 
the blood stream. 
m. THEORIES OF IMMUNITY. 
Two distinct explanations have been advanced to account for the 
mechanism of immunity as it is observed diu'ing the course of disease: 
the cellular or phagocytic theory championed by Metchnikoff and 
his followers, and the humoral theory developed by Ehriich. 
Neither theory affords a satisfactory explanation of all the features 
of immunity following infection and it is very probable that cellular 
activity and the production of specific antibodies is more important 
in certain types of infection (notably in association with the exogenous, 
pathogenic bacteria), while phagocytic activities are more intimately 
> Hektoen: Jour. Am. Med. Assn., 1900, 46, 1407. 
