MICROBES— INFECTION— IMMUNITY 457 



having to pass through the body of an intermediate host, 

 and a still further complication exists when it is not the 

 intermediate host but only its progeny which can give the 

 disease. As an example of direct inoculation we may 

 quote anthrax or glanders ; of infection through an inter- 

 mediate host we have trypanosomata diseases ; while of 

 infection through the progeny of the intermediate host we 

 have all the piroplasmoses. 



Immunity. 



Immunity is the ability of an individual or species to 

 resist infection. The resistance offered may either be 

 against the organisms or against their toxins. 



Immunity is natural or acquired. Natural immunity is 

 either complete or relative; acquired immunity may be 

 active or passive. There are two modern theories which 

 endeavour to explain the interesting practical phenomenon 

 of immunity. 



A. Antitoxic Serum. 



1. Ehrlich's Lateral Chain Theory. — An animal sus- 

 ceptible to a certain toxin, may be made highly immune by 

 gradually increasing the quantities of toxin injected, and 

 the blood serum of such an animal by this process of 

 hyper-immunisation, acquires properties which neutralize 

 the action of the toxin ; the serum is therefore termed 

 antitoxic, and the antitoxic effect on the poison can be 

 produced either within the body or in vitro. 



It is believed that the neutrahzation thus produced is a 

 chemical one, for it follows the law of chemical combina- 

 tion that certain definite amounts of antitoxin are required 

 to neutralize certain definite amounts of toxin. 



Ehrlich considers that a toxin molecule possesses two 



groups. 



(1) A stable group described as the haptophore, which 

 attaches itself to a certain unstable or lateral group in the 

 animal cell (Fig. 187, p. 460). 



(2) An unstable group known as the toxophore (Fig. 187), 



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