ANTITOXIC IMMUNITY. 155 



may, or may not, contain antitoxin. If the resist- 

 ance is referable to the presence of antitoxin, the 

 latter may be detected in the following manner: 

 The animal is bled, its serum collected from the 

 clot, then mixtures of the serum and of the toxin 

 are injected into animals of known susceptibility 

 for the toxin. If the test animal is in this way 

 protected from an otherwise fatal dose of the toxin, 

 it is evidence that the serum contains an antitoxic 

 substance. On the other hand, if the serum shows 

 no such antitoxic effect, we must conclude that the 

 resistance of the animal is due to other causes; as, 

 for example, non-susceptibility of the tissues, 

 power of the living cells or ferments to destroy the 

 toxin, or absorption of the toxin by tissues of sec- 

 ondary importance to life. 



Following this method of experimentation, if 

 antibacterial properties are found to the exclusion 

 of antitoxic, the immunity is considered to be an- 

 tibacterial ; and with the converse result it is anti- 

 toxic, or dependent on non-susceptibility. It is, of 

 course, conceivable that in a given case it might be 

 both antitoxic and antibacterial. In dealing with 

 diseases of which the specific microbe is known 

 and cultivated, the existence of antibacterial or of 

 antitoxic substances can usually be found by the 

 methods described. If the .etiology is unknown, or 

 the micro-organism cannot be cultivated, as in 

 scarlet fever, measles, etc., that is, if the virus and 

 its toxin cannot be obtained in quantities, the 

 nature of the resistance is not at present open to 

 determination. 



It is seldom that natural resistance is absolute. 

 Pasteur found that the great immunity of the 

 chicken for anthrax could be overcome by im- 



