256 GENERAL BIOLOGY 



The latter is the basis of the " anti-toxins " that are 

 now so extensively used in combating infectious 

 diseases. It is only the active immunity that con- 

 cerns us here. 



When disease-producing bacteria gain entrance to 

 the body and begin to multiply, the products of their 

 metabolism produce a poisoning or " toxic " effect, 

 either upon the whole organism or upon certain 

 organs, such as the nervous system. The body 

 doubtless reacts in several ways. Thus it has been 

 shown that the leucocytes devour the invading 

 microbes and thus defend the organism from attack. 

 But the most striking phenomenon is this : that the 

 presence of the toxins stimulates the body to produce 

 substances called " anti-bodies," which are carried 

 by the blood and appear to combine with the toxins 

 and nullify their poisonous effect. Once having been 

 stimulated, the body continues to produce the anti- 

 bodies after all disease-symptoms have disappeared. 

 In this way, it is immune against a second attack, 

 whereas it was susceptible before. 1 Moreover, 

 the blood-serum carrying these anti-bodies may be 

 drawn from one animal and injected into another, 

 thereby conferring the passive immunity mentioned 

 above, 



Morphogenetic Response. In the previous sec- 

 tion, with a few exceptions, we have considered the 

 reactions of the individual to stimuli that are, as a 

 rule, of short duration, or at any rate produce no 



1 In the human species it will be recalled that there are some diseases 

 of which this is not true. 



