﻿MODERN THEORIES IN RELATION TO IMMUNITY. 75 



Exactly the same view applies to conferred immunity. A natural 

 immunity may be increased or one which is scarcely existent may be 

 rendered apparent and protective by the introduction of cells, or the 

 products of those cells. In these cases we produce a more or less specific 

 type which manifests itself either only against the particular cell in 

 question or at least only against those also with which it is closely 

 related. This conferred immunity, in principle, does not differ from the 

 hereditary one; we have only added to the host another set of chemical 

 substances which in themselves were not present before, or at least 

 were present only in small amount. However, bodies related to the 

 introduced substances, or at any rate bodies having similar chemical 

 characteristics must have existed in the host owing to the principles of 

 heredity which have jiist been discussed and it must be by reason of this 

 similarity or identity that the increased immunity is conferred. The 

 introduced cell then, by reason of its introduction into a host, causes a 

 disturbance of equilibrium in certain cells of the latter, these cells, ac- 

 cording to the law by which a chemical system tends to produce a change 

 in the surrounding conditions by which the disturbance can be counter- 

 acted, give off or produce chemical substances which tend to destroy or 

 eliminate the invader and in so doing they, responding to the stimulus, go 

 far beyond the point of original equilibrium and so produce the immune 

 substances which find their way into the blood. However, the laws of 

 chemical equilibrium will prevent too great a proliferation of these im- 

 mune bodies, for there must come a point, if we consider physical laws, 

 when the reaction which produced the immune bodies takes place in the 

 opposite direction, and at this point as many of the immune bodies will be 

 destroyed as created. To speak in the language of chemistry, the reaction 

 must be a reversible one. This condition must arise, or otherwise we could 

 go on in any given case to a condition of infinite immunity. How this 

 reversal is brought about is a matter of indifference. It may be by the 

 formation of anti-immune bodies, it may be by a process destructive 

 to immune bodies which is constantly going on within the cells of the 

 host ; in any event the end of the equilibrium must be reached. This new 

 end equilibrium, however, in the case of the conferred immunity, is only 

 an unstable one, the organism must gradually tend to return to its 

 original condition; that is, the conferred immunity must gradually be 

 lost, the rapidity of the change depending upon the nature of the 

 immunity which has been given. Conferred immunity, therefore, differs 

 from natural or hereditary immunity by not being permanent, for in the 

 hereditary type the equilibria have been established through the ages 

 during which the permanent condition has gradually been brought 

 about — the system has adjusted itself to the new equilibrium. It would 

 be interesting to study the production of hereditary immunity in animals, 

 but I fear the task would be a great one, for, as we know, hereditary 

 variations in type are only produced by many generations. 



