INFECTION AND IMMUNITY. 549 



been protected or the bacteria that produced it. Such 

 passive immunization is not in general possible with the 

 serum from animals rendered immune from endotoxins. 



THE DEFENSES OF THE BODY. In so far as we 

 now know the means of defence used by the body in 

 its warfare against infective bacteria and their poisonous 

 products are the phagocytic cells, such as the leucocytes, 

 the large mononuclear cells of the blood, and the con- 

 nective tissue and endothelial cells, as well as ill-defined 

 vital substances in the circulating blood which act, so 

 to speak, as antidotes to bacterial poisons. If the 

 defenses are not of sufficient vigor to destroy the in- 

 vading bacteria, or to render inert the poisons produced 

 by them, the bacteria are victorious and infection results ; 

 on the other hand, if there be failure to excite disease, 

 the tissues are victorious, and are then said to be resistant 

 to or immune from this or that particular type of infection. 



In some cases the protective agents possessed by the 

 animal act directly upon the invading organisms them- 

 selves i. e. 9 they are germicidal ; in others their func- 

 tion is more that of antidotes, or neutralizers in the 

 chemical sense, of the poisons produced by these organ- 

 isms, the organisms themselves, in certain instances, 

 experiencing only slight injury from a limited sojourn 

 in the living tissues. Careful investigation has shown 

 that the normal bactericidal properties of the blood 

 serum rest upon the presence of substances similiar in 

 nature to those found in the blood serum of artificially 

 immunized animals. 



So far as we can learn the blood serum contains normally 

 a small amount of antitoxic, agglutinative, and bacteri- 

 cidal aotion against a great variety of pathogenic bacteria. 



