254 ACQUIRED IMMUNITY 



424. The outline I have attempted to give of the process of 

 digestion is, of course, even if correct in all its facts and inferences 

 a mere sketch. Each species of bacteria may secrete not only one 

 but two or more toxins complementary to one another. Thus 

 there is ample evidence of the existence in acute diseases of toxins 

 which are retained within the microbes, in addition to those which 

 are passed into the surrounding medium. Probably the organisms 

 of the chronic diseases (e.g. tuberculosis) are protected more especi- 

 ally by the former, those of the acute diseases (e.g. small-pox) by the 

 latter, whereas those of the sub-acute diseases (e.g. syphilis) depend 

 in a more equal degree on both. It is significant that an extract 

 obtained from dead tubercle bacilli is very poisonous, while one simi- 

 larly obtained from diphtheria bacilli is not. Again, there is some 

 evidence that one kind of enzyme acts directly against the microbes 

 and another against their toxins. The cells, therefore, may secrete, 

 not one kind of enzyme, but perhaps several. The same enzyme 

 may not protect against every disease. The process of digestion 

 may not be continuous, but may proceed by well marked steps in 

 which various cell products, globulins, lysins, alexines, opsonins, 

 and the like successively take part. As far as I am able to judge, 

 however, no matter how complicated the process, it is somewhat of 

 the nature I have attempted to describe. 



425. Inborn immunity on the other hand must in each case be 



have some curative properties, which depend apparently on an increase of in- 

 flammation round the foci of infection which leads to necrosis and sloughing of 

 the diseased tissues ; but there is no evidence that it conduces to that toleration, 

 that acquired immunity, to the attainment of which the antitoxic treatment of 

 acute disease is so great a help. The antitoxic treatment of such troubles as 

 alcoholism and morphomania has also been tried with declared success by the 

 originators of the treatment, with ill-success by all who followed them. Doubt- 

 less serum treatment of a tendency to fatigue by means of an antitoxin obtained 

 from very tired animals will one day be attempted, and will meet with the same 

 unsatisfactory sort of success. Some of the hypotheses formulated and acclaimed 

 by bacteriologists are of a kind to fully justify the endeavour. The speed with 

 which immunity is acquired bears an evident relation to the abundance of the 

 extra-cellular toxins and the rapidity with which they are produced by the 

 microbes. All the cells of the body are affected by them and all those which are 

 functionally concerned in the acquirement of immunity tend to react. In the 

 case of the intra-cellular toxins only those cells which ingest the microbes are 

 affected. Possibly this is the reason why immunity is acquired in the one case 

 and not in the other. The duration of immunity, which is usually life-long in 

 some diseases and of short duration in others, depends on factors which are quite 

 unrecognized. It is sometimes said that one attack of certain diseases (e.g. 

 diphtheria) in which the period of immunity is short, predisposes to subsequent 

 attacks ; but the truth of this surmise is very doubtful. An attack of such a 

 malady indicates that the individual is susceptible ; subsequent attacks probably 

 indicate no more than a recrudescence of this susceptibility. 



