136 THE DIFFERENT TYPES OF IMMUNITY 



to small doses of the toxin, but absence of immunity to a larger 

 dose, the result, moreover, varying with the point at which the 

 toxin is introduced. If this should occur in a territory which con- 

 tains large numbers of insusceptible cells (though provided with 

 suitable haptophoric groups) no deleterious results woukTbe expected, 

 while in the opposite case the consequences would of necessity be 

 disastrous. A great deal, moreover, other things being equal, would 

 depend upon the size of the dose, for if this should exceed the demands 

 of the insusceptible cells a toxic effect would naturally be the out- 

 come. In such instances, then, the immunity w r ould only be relative. 

 This is exactly what we see in the case of the rabbit, which is 

 relatively insusceptible to tetanus toxin, when this is administered 

 hypodermically, but highly sensitive if the poison is injected directly 

 into the brain. 



It will be noted that these three different types of immunity are 

 thus essentially dependent upon the character of the cells, i. e., 

 that they are histogenetic in character, and that the examples which 

 have served as illustrations at the same time represent types of 

 natural immunity. But we have also seen that immunity, sc., 

 increased resistance, must result if for any reason antitoxin mole- 

 cules enter the circulation in sufficient number to neutralize any 

 toxin that may be present. Immunity of this order is thus humoral 

 in character and usually, if not always, acquired. This may result 

 as a consequence of infection or immunization, and then represents 

 a type of active immunity, or it is acquired in a passive manner, 

 the organism of the individual taking no part in its production 

 (passive immunity). An example of the first type is furnished by the 

 antitoxin horses, in which a high degree of immunity is produced 

 by systematic immunization, while the production of passive immu- 

 nity is illustrated in the prophylactic treatment of diphtheria or 

 tetanus with the corresponding antitoxic sera. To the latter order 

 also belongs the immunity which is conveyed by actively immunized 

 animals to their offspring, either during intra-uterine life or post 

 partum through the milk. 



While the underlying principle of these types of immunity is thus 

 quite well understood, still another form of acquired immunity is, 

 theoretically at least, possible. We have seen that under natural 

 conditions a form of antitoxin immunity exists, which is referable 

 to absence of suitable haptophoric groups on the part of the body 



