DISEASE IN GENERAL 141 



ity of infections are brought under control by the action of anti- 

 bodies before they have done any harm, as the number of infecting 

 organisms is usually small. On the other hand, if the invasion of 

 bacteria is great the antibodies are not capable of overcoming them, 

 and disease results either from the mechanical interference pro- 

 duced by the organisms or from a poisoning of the body by the 

 noxious bacterial toxins which circulate with the blood to all parts 

 of the body. Certain organs are more susceptible to the action of 

 these poisons than others and give way to them much quicker. 

 This is nicely illustrated in cases of tetanus, where the organs of 

 the nervous system are the first to show the effects of the toxin 

 produced by the tetanus bacilli. Among the most useful anti- 

 bodies are antitoxins, which neutralize the harmful effect of bac- 

 terial toxins; agglutinins, which cause bacteria to clump or ag- 

 glutinate and lose their motility; precipitins, which precipitate 

 certain organisms under favorable conditions. 



II. The Phagocytes or White Cells of the Blood.— These cells 

 are capable of passing through intact walls of the blood-vessels. 

 They collect at the seat of infection and take up and destroy the 

 bacteria that are attempting to invade the tissues. As active 

 agents in aiding the body in the fight against foreign invaders they 

 are very important. 



III. The Secretions of Certain Glands. — These possess marked 

 bactericidal properties. A good example is seen in the germicidal 

 action of the acid gastric juice. 



IV. The Elevation of the Body Temperature. — This reaction 

 follows an invasion with bacteria, and assists in eliminating them 

 by oxidation or by making their surroundings unsuitable for 

 growth. 



V. The Intact Skin and Mucous Membranes. — Both these 

 structures belong to the epithelial tissues, and are capable of ex- 

 cluding most micro-organisms and of serving as efficient external 

 defences against invaders from the surfaces of the body. 



Immunity is the general term used to express all of the factors 

 concerned with the natural resistance of the body against disease 

 which have just been mentioned. It may be defined as that con- 

 dition in which an individual or a species of animals exhibits resist- 

 ance to an infection for which other individuals or other species 

 show a greater or less degree of susceptibility. So far as is known, 

 immunity is possible only against those diseases that are caused by 



