260 BACTERIOLOGY 



of the gastric juice, low though it be, is thought sufficient 

 to render inert certain infective bacteria that enter the 

 aUmentary tract by way of the mouth. Of all the defenses, 

 however, none are certainly of so much importance as those 

 to be detected within the internal structures of the normal 

 animal. In its warfare against the invasion of infective 

 bacteria and the activities of their poisonous products, the 

 most significant defenses possessed by the body are those 

 which directly aim at the destruction of the living germs of 

 disease and at the neutralization of their poisonous waste 

 products. 



In so far as we now know the internal means of defense 

 used by the body in its warfare against infective bacteria 

 and their poisonous products are the phagocytic cells, such 

 as the leukocytes, the large mononuclear cells of the blood, 

 and the connective tissue and endothelial cells, and the 

 ill-defined vital substances in the circulating blood which 

 act, so to speak, as antidotes to bacterial poisons. If these 

 defenses are not of sufficient vigor to destroy the invading 

 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 

 have been 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 organism act directly upon the invading parasites 

 themselves — i. e., they are germicidal; in others their 

 function is more that of antidotes, or neutralizers in the 

 chemical sense, of the poisons produced by these parasites, 

 the parasites themselves, in certain instances, experiencing 

 only slight injury from a limited sojourn in the living tissues. 



So far as we can learn the blood serum exhibits normally 



