270 MEDICAL BACTERIOLOGY 



substances. The same is true of cellular digestion. Some cells possess recep- 

 tors which manifest an equal or greater affinity for certain poisons than for 

 food ; if the blood serum brings such poison to a cell so equipped it is assimilated 

 and as a result one of two possibilities result: the cell dies, or it is injured, the 

 receptors uniting with the poison exhausted, the cell temporarily arrested in 

 its activities; the arrest stimulates greater activity on the part of the cell, 

 and as it recuperates the cell not only reproduces the destroyed parts, but 

 forms many more receptors than were exhausted, so many, that some 

 are cast off and taken up by the blood serum. If the blood serum receives 

 more of the poison, the free receptors in the serum unite with it and precipitate, 

 agglutinate, neutralize or disintegrate it, so preventing the poison from reach- 

 ing and injuring the cell the cell is immune so long as the serum contains 

 free receptors. 



Ehrlich showed that these receptors or antibodies are divisible into three 

 groups: receptors of the first order, receptors of the second order and receptors 

 of the third order. There are certain properties common to all three and others 

 peculiar to each group. All are products of cellular activity, all occur in blood 

 serum, all are to a degree specific in action uniting with a single substance, all 

 may be increased up to a certain point by stimulation of the cells producing 

 them. 



Receptors of the first order include the antitoxins. In the blood serum they 

 are almost or entirely confined to the pseudoglobulin portion. They are 

 relatively stable, serum containing them showing a loss of about i per cent, or 

 less of its receptor content, for months after removal from the body, if kept 

 sterile and in a cool, dark place (ice box). An exposure of 6oC. to 8oC. for 

 % hour destroys them. 



Receptors of the second order include the agglutinins and precipitins and 

 these are more complex than receptors of the first order having two distinct 

 parts, one part which unites with the substance to be agglutinated and another 

 part that produces the agglutination. Receptors of the second order remain 

 active in serum for months after removal from the body if kept cool and sterile. 

 An exposure of ^ hour to 7oC. permanently destroys receptors of the second 

 order. 



Receptors of the third order, the most complex of all, include the lysins and 

 have been previously described (page 267). 



DISCUSSION OF THEORIES OF IMMUNITY 



The theories of Metchnikoff and of Ehrlich both recognize that active im- 

 munity is dependent upon cellular activity, and exaltation of it is the result of 

 stimulation (irritation) of the cells by the poisonous substance against which 

 immunity is enhanced. 



The recognizable mechanism of infection and immunity amply substantiate 

 this. 



Numerous observations indicate that the mechanism of infection, resistance 

 and immunity is in some cases substantially as explained by Ehrlich; in others 



