THEORIES OF IMMUNITY 255 



The student will better understand this theory if he 

 recalls his fundamental physiology. I^iving substance is 

 characterized, among other things, by irritability, which is 

 instability. It is in a constant state of unstable equilibrium. 

 Whenever the equilibrium becomes permanently stable the 

 substance is dead. It is also continually attempting to 

 restore disturbances in its equilibrium. Whenever a chemi- 

 cal substance unites with a chemical substance in the cell, 

 a receptor, the latter is, so far as the cell is concerned, 

 thrown out of function for that cell. The chemical equi- 

 librium of the latter is upset. It attempts to restore this 

 and does so by making a new receptor to take the place 

 of the one thrown out of function hence the chemical combina- 

 tion betw^een the chemical substance introduced and the cell 

 receptor is the stimulus tvhich causes the cell to make a new 

 receiAor. If this process is continued, i. e., if the new receptor 

 is similarly "used up" and others similarly formed are also, 

 then the cell will prepare a supply of these and even an excess, 

 according to W^eigert's theory. Whenever a cell accumulates 

 an excess of products the normal result is that it excretes them 

 from its own substance into the surrounding lymph, whence 

 they reach the blood stream to be either carried to the true 

 excretory organs, utilized by other cells or remain for a 

 longer or shorter time in the blood. Hence the excess of 

 receptors is excreted from the cell that forms them and they 

 become free in the blood. These free receptors are termed 

 antibodies. They are receptors, but instead of being retained 

 in the cell are free in solution in the blood. One function 

 of the free receptor, the antibody, is always to unite with 

 the chemical substance which caused it to be formed. It may 

 ham additional functions. The chemical substance which 

 caused the excess formation of receptors, antibodies, is 

 termed an antigen for that particular kind of antibody. 



To recapitulate, Ehrlich's theory postulates specific chemi- 

 cal stimtdi, which react with specific chemical substances in 

 the body cells, named receptors, and that these receptors, 

 according to Weigert, are produced in excess and hence are 

 excreted from the cell and become free receptors in the blood 

 and lymph. These free receptors are the various kinds of 

 antibodies, the kind depending on the nature of the stimulus, 



