ACQUIRED IMMUNITY 117 



cannot be acquired. Evidently, therefore, toxins and immunity 

 are related, more or less immediately as cause and effect. 1 

 The quickness and completeness with which immunity may 

 be produced against any disease seems, generally speaking, 

 in direct relation to the quickness with which the toxins are 

 produced and their degree of virulence. Thus immunity is 

 quickly acquired against such diseases as measles and small- 

 pox, but more slowly against enteric fever or syphilis. Malaria 

 and relapsing fever are apparent exceptions to this rule, for 

 though their toxins are powerful and quickly produced, yet 

 these diseases are of long duration. It is possible, however, 

 that the periods of intermission which distinguish these mala- 

 dies are really periods of acquired immunity an immunity 

 which is lapsed very soon after it is acquired. The parasites 

 are then absent from the blood, or occur in a highly-resistant 

 but non-virulent form. 2 The duration of immunity bears 

 some relation to the virulence, or at any rate to the abund- 

 ance of the toxins, for a severe attack of small-pox pro- 

 tects for a longer time than an attack of modified small-pox 

 (vaccinia). But this, while it may be true of attacks of the 

 same disease, is not true of attacks of different diseases. Thus, 

 chicken-pox, a comparatively mild disease, usually confers 

 lasting immunity, while diphtheria, a much severer disease, 

 does not. 



190. What is the nature of acquired immunity? What 

 precisely are the effects of the change which a man under- 



1 If the above be correct, the antitoxic treatment of tuberculosis 

 attempted by Koch and others in effect an attempt to procure acquired 

 immunity is, from the nature of the case, doomed to failure. Apparently 

 the bacillus tuberculosis produces no powerful toxins against which 

 immunity may be acquired. An individual may suffer for any number 

 of years from a pure tuberculosis, as of joints and glands, without 

 becoming in the least immune. A toxic intercellular substance has been 

 extracted from the bacillus which is useful for diagnostic purposes, but, 

 evidently, it is of a nature quite different from the true toxins which 

 are secreted into the surrounding medium by such micro-organisms as 

 those of diphtheria and tetanus. For the same reason an antitoxic 

 treatment of leprosy and cancer supposing it to have a microbic origin, 

 which is extremely doubtful should be impossible. 



2 In malaria the period of intermission is produced not so much by 

 the absence of the parasites as by the absence of the toxins. The reader 

 can hardly have realized as yet the part that Natural Selection has 

 played in the evolution of these low forms ; but it is not impossible 

 that the non-virulent stage the period of the formation of resistant 

 spores is entered upon by the protozoon as a means of protecting itself 

 against phagocytes which have acquired the temporary power of tolerating 

 the toxins and attacking the microbes. A more permanent immunity is 

 certainly very slowly acquired. 



