374 BIOLOGY: GENERAL AND MEDICAL 



out an ordinary lifetime. Our parents probably suf- 

 fered from the same affections, but we did not profit by 

 their sufferings, and our children shall not profit by ours. 

 Most of us have been vaccinated and thereby acquired 

 immunity against small-pox, but do not transmit it 

 to our descendants. 



Acquired immunity, therefore, appears to be some- 

 thing within the province of experiment and about 

 which much may be learned. It is also a practical 

 matter, for means by which immunity against the infec- 

 tious diseases may be acquired and these diseases pre- 

 vented is of the utmost importance to society. 



By what means can immunity be acquired? The 

 answer to this question is, by habituation. In discussing 

 the elementary characteristics of living matter it was 

 shown that reaction to stimulation becomes less pro- 

 nounced the more frequently the stimulations are 

 repeated, provided such stimulations are not of an inten- 

 sity injurious to the protoplasm or produced by stimuli 

 destructive in quality. In many cases the failure of the 

 irritable response is due to fatigue or exhaustion; in 

 other cases it may be due to habituation. That is, 

 the stimulant having proven less harmful than at first 

 appeared, an adjustment is effected by which subse- 

 quent contacts produce diminishing effects. This habit- 

 uation of the cell to repeated stimulation and the con- 

 sequent diminution of the reaction may be fundamental 

 in explaining acquired immunity. 



The phenomena of immunity embrace two different 

 series of reactions, one of which is directly referable to 

 the cells, which participate actively, the other indirectly 

 referable to the cells. The former are directed toward 

 the destruction of the organized living entities micro- 

 parasites, the latter toward the destruction of their 

 chemical poisons (toxins). 



Let us first consider the reactions of intoxication. 

 It is well known that by frequent administration and 

 cautious increase in the dosage, tolerance can be estab- 



