198 GENERAL BIOLOGY 



affinity for various substances that pass through the body. This mole- 

 cule is called a receptor. We know that normally, as blood passes the 

 different cells of the body, the cells have a selective action, that is, they 

 practically reach out and drink in what they need. One of the experi- 

 ments performed on Pararnoecia demonstrates what is mea,nt by this 

 chemical selective action. It is there shown that certain chemical sub- 

 stances such as a sugar solution may cause the animal to go in an oppo- 

 site direction, but if it has once gone into the solution it will not again 

 leave. This selective action which all cells probably possess to some 

 degree, may work on a similar basis ; that is, normally, the molecule 

 (the receptor) draws to itself the particular food that it needs as the 

 blood passes. But, just as Pararnoecia may actually enter the sugar solu- 

 tion or even various injurious solutions, so the molecule or receptor may 

 also sometimes take or select from the passing blood poisonous or toxic 

 substances and unite them with itself. This, of course, injures the cell 

 to which the receptor is attached. 



We know from ordinary observation that whenever we injure our- 

 selves sufficiently, a scar forms. Then, too, it will be noticed that the 

 scar is almost always slightly elevated. This means that more scar 

 tissue has actually formed than there was skin before. From micro- 

 scopic studies we find that whenever those particular cells known as 

 fibroblasts (which form a goodly portion of the connective tissue ele- 

 ment of the body) are injured, they grow much more rapidly and pro- 

 fusely than they did before such injury took place; in other words, if the 

 fibroblasts are injured, more connective tissue will grow in the region 

 of injury than grew originally. Once an injury takes place and regen- 



Oeration or regrowth begins, there 

 i "l^ c: "***^\ * s usually an excess of such re- 



l j-J ft ._ jJr generation or of such growth. 



With this in mind it is easy 

 d. a. c. to understand that when a mole- 



Fig. 91. Diagram Illustrating the Factors cule Or TCCCptOr has anchored t6 



Concerned in Immunity. . 1f . /-,-,. r\i \ i i 



cl.. the cell to be dissolved; c., the com- *tself a P OlSOn (Fig. 91) which 



plement or solvent by which it is dissolved; injures the Cell tO which the FC- 

 a., the amboceptor or intermediate body by 



which the two can be brought together. CCptOr IS attached, the Cell 



grow several receptors where it had only one before. Such excessive 

 production of receptors causes a portion of the receptors to be thrown 

 off from the cell. These separated receptors then find their way into 

 the blood-stream. The receptors in the blood-stream are able to anchor 

 poisons to themselves just as when attached to cells. This means that 

 there are great quantities of these receptors taking up the poison that 

 would otherwise injure the various cells with which the poisons might 

 come in contact. The receptors thus prevent injury to cells which nor- 

 mally would be open to attack. 



Certain conditions, however, must be fulfilled before the receptors 



