NATURAL IMMUNITY 59 



slight individual variations. In fact, the astonishing uniformity of 

 reaction on the part of guinea pigs of similar age and weight against 

 measured quantities of bacterial toxins has alone made it possible 

 to utilize these animals in the standardization of antitoxins. Pneu- 

 mococcus and streptococcus cultures can be measured with reason- 

 able accuracy upon white mice of approximately uniform weight, 

 and the same animals are relatively uniform in their reactions to 

 identical amounts of tetanus poison. Many other examples might 

 be cited which make it clear that healthy animals of the same species, 

 kept under the same conditions, fed upon the same food, and of ap- 

 proximately the same age and weight, differ but slightly from each 

 other in reaction to the same infectious agent. This would indicate 

 that the individual differences in resistance displayed so plainly by 

 human beings are due, not to any fundamental individual variations, 

 but rather to such fortuitous factors as nutrition, metabolic fluctua- 

 tions, temporary physical depression, fatigue, or chilling. A person 

 suffering from functional impairment of any kind is more likely 

 to permit the invasion of a pathogenic micro-organism than is a per- 

 fectly healthy well-nourished individual of the same species. 



Most of these facts we know from the accumulated experience of 

 clinicians who also have given us much valuable information con- 

 cerning the susceptibility to infection on the part of chronically dis- 

 eased persons, especially diabetics and nephritics. In the case of a 

 few of these influences, chilling and fatigue, experimental data on 

 animals are available. It is, however, extremely difficult to analyze 

 the causes underlying such depression of resistance. For instance, 

 with fatigue or chilling there may be temporary congestion of mucous 

 surfaces, due to vasomotor influences, which alter the secretions on 

 mucous surfaces, or interfere with the normal mobilization of 

 leukocytes, permitting penetration of bacteria where ordinarily 

 they would have been held back. Our ignorance is nowhere more 

 clearly illustrated than in the fact that we know practically nothing 

 concerning the relation between a thorough chilling and the acquisi- 

 tion of what is spoken of as a common "cold." We can only assume 

 that there is interference in some way with the normal bactericidal 

 and phagocytic mechanisms, making possible the penetration and 

 lodgment of small quantities of bacteria, ordinarily destroyed imme- 

 diately after entrance or prevented from entering at all. 



Of course we must except those individual differences of sus- 

 ceptibility which may be dependent upon inheritance. We know, 

 for instance, that in such diseases as diphtheria, where resistance 

 depends upon antitoxins circulating in the blood, there may be a 

 passive immunity, conferred from mother to offspring, which lasts 

 for several weeks or months after birth. It is important to remem- 

 ber such a possibility in the selection of guinea pigs for diphtheria 

 antitoxin standardization, as Anderson has pointed out. Whether 



