HOW THE BODY FIGHTS DISEASE 



change, where a gay sort of Virginia reel is going 

 on, the atoms, or rather groups of atoms, flinging 

 off their partners and taking new ones with the 

 precision and dexterity of a practised band. But 

 woe if some clumsy or evil-minded disturber comes 

 in to break up this rhythmic play. For the price 

 of life is that this molecular dance shall not for 

 one briefest instant cease or swerve. 



The poisons are the disturbers. Though they be 

 all in faultless attire, resembling the other guests 

 so closely that no police will step in to show them 

 the door, something in their disposition, some evil 

 ineptitude, brings ruin in their train. 



Perhaps this is carrying a fanciful analogy too 

 far. A great German chemist makes use of the 

 simile of a lock and key. Here the microscopic cells 

 of which the body is composed may be pictured as 

 furnished with many doors, these doors with locks ; 

 in ordinary normal conditions the doors swing 

 wide, the traffic of the body goes on without let or 

 hinderance. But suppose a structure so adapted as 

 just to fit the lock or release a spring the door is 

 closed, traffic stops. 



But lively pictures of this sort have the incon- 

 venience of being a little too concrete that is to 

 say, they imply things which do not tally with all 

 the facts. In the case of infection and the process 

 of immunization the facts seem simple and toler- 



283 



