Immunity 199 



that is, they practically reach out and drink in what they need. One 

 of the experiments performed on Paramoecia demonstrates what is 

 meant by this chemical selective action. It is there shown that certain 

 chemical substances, such as a sugar solution, may cause the animal to 

 go in an opposite 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 Paramoecia may actually enter 

 the sugar solution or even an injurious solution, so the molecule or 

 receptor may also sometimes take or select from the passing blood 

 various 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 -*-i c: • t00 * *\ * s usually an excess of such re- 



l_ _P P / generation or growth. 



With this in mind it is easy 

 cL °* to understand that, when a 



Fig> 91 \ ctSdiS2t y ^ Factors molecule or receptor has anchored 



cl., the cell to be dissolved; c, the com- to itself a poison (Fig. 91) which 



plement or solvent by which it is dissolved; ininrPQ trip pp11 tn whirri trip rp- 



a., the amboceptor or intermediate body by injures tne Ceil IO WniCU tne re- 



which the two can be brought together. ceptor is attached, the cell may 



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 which 

 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 

 can unite to themselves the poisonous substance, and the condition 

 necessary in this instance is that a certain ferment-like substance, called 



