10 INTRODUCTION 



immunity. A toxic molecule may be assimilated through a special 

 receptor, either damaging or destroying the cell ; but if there exist 

 no receptors that have an affinity for a certain toxin, the cell is 

 immune to that special toxic molecule. A toxic molecule is supposed 

 to have two groups of affinities : i. a toxophorous group, which does 

 damage to the cell ; 2. a haptophorous, or combining group, which 

 had an affinity for its special receptor. Figure 8 represents diagram- 



matically a toxic molecule. The shaded 

 , portion is the toxophorous group, and the 



croup clear portion, the haptophorous group. 



These toxic molecules may unite with their 

 special receptors and be thrown off by the 

 Haptophorous ce ll into tne blood-current. When this 

 group occurs, the toxins do not reach the cell; 



1 I but their affinities are neutralized and they 



become innocuous. If, however, the toxic 



Fig. 8. Diagram of a toxic molecule. , , . , ._ 



molecules are too abundant to be thus dis- 

 posed of, they may enter the cell through the appropriate receptors 

 and destroy it. The throwing off of combined receptors and toxins in 

 this way is a protective process, but it is limited. 



Receptors. When cells are attacked by toxins, there is a great 

 over-production of receptors, and many of them are thrown off without 

 having formed a union with toxic molecules. Thus, receptors float- 

 ing in the blood still have an affinity for special toxins, which they 

 break up and render innocuous. In this way they protect the cells, 

 acting as antitoxins. This process is roughly illustrated in Figure 9. 



Passive and Active Immunity. What is known as passive immu- 

 nity is secured by introducing into the body of an animal a serum 

 containing a protective substance that acts as an antitoxin. The anti- 

 toxin consists of receptors that have been formed in excess and thrown 

 off into the blood-serum, which thereby becomes antitoxic. These 

 antitoxic receptors combine with and neutralize the toxins. Passive 

 immunity is produced promptly but soon passes away. It is called 

 passive immunity for the reason that the immunizing bodies have been 

 produced by another animal, the animal thus rendered immune being 

 passive and simply receiving the immunizing receptors. Active immu- 

 nity is a much slower process and is more lasting. In this form of 

 immunity the animal produces its own antitoxin. Receptors unite with 

 toxic molecules, are thrown off into the blood, become innocuous and are 

 eliminated through the emunctories. In addition, new receptors (anti- 

 toxins) are produced in large excess (over-production) and are thrown off 

 by the cells into the blood. They thus exist in the blood, ready to neu- 



