THE DEFENSES OF THE BODY 259 



immunity, but the substance appearing in the blood of 

 animals so immunized is rather bacteriolytic, and the blood 

 of such animals may contain little or no true antitoxic 

 components. 



INIoreover, if the blood serum of an animal immune from 

 true toxin be injected into a normal animal, this latter at 

 once acquires some degree of resistance to the toxin from- 

 which the first animal was protected, i. e., it is "passively" 

 immunized; on the other hand, if the bacteriolytic serum of 

 an animal artificially immunized from endotoxic bacteria 

 be similarly transferred to a normal animal, there is no similar 

 transference of the state of immunity. 



THE DEFENSES OF THE BODY. 



When considered in the most comprehensive way we find 

 that the normal body is endowed with a number of natural 

 provisions that may fairly be regarded as defenses against 

 the invasion of hurtful parasites. Thus for instance: If 

 the skin of even the most cleanly persons be examined 

 bacteriologically, we find that in the majority of cases bac- 

 teria of several kinds, often those having the power to cause 

 disease, are to be detected. So long as the skin is intact 

 and the individual in good general health no harm results. 

 The reason for this is found in the structure of, the skin. 

 The horny epidermis and the fat and sweat secretions serve 

 as effectual barriers against both the multiplication of germs 

 and their penetration into the underlying tissues. The hairs 

 about the orifices act to some extent as filters or screens for 

 bacteria laden dust; the ciliated epithelium of the upper air 

 passages serves as a sweep to rid the body of foreign par- 

 ticles that may find lodgment upon it; and the acid reaction 



