ACCIDENTAL AND EXPERIMENTAL IMMUNITY. 117 



germicidal property of the blood, yet it may, to some extent, 

 be held to account for it. 



Immunity to the toxins, according to Ehrlich, is due to the 

 fact that some of the tissue-cells are capable of forming 

 chemical combinations with the toxin by means of so-called 

 molecular side-chains ; that is, that these substances, or side- 

 chains, neutralize the toxin and thus allow the " natural resist- 

 ance " of the system at large to dispose of the germs. This 

 theory will be discussed more fully later on (see page 122). 



Emmerich believed that bacteria produce enzymes which are 

 capable of digesting not only the germs that caused their for- 

 mation (con forme), but also in some cases other germs as well 

 (heteroforme). The immune serum contains more enzymes 

 than do artificial cultures of bacteria. These enzymes may 

 account also for the degeneration-forms of bacteria seen in 

 old cultures, and for the self-limitation of infectious diseases. 

 He calls these enzymes nucleases. In order to distinguish 

 them, each enzyme is named after the organism producing 

 it, as ty phase, pyocyanase, etc. An immune proteid, accord- 

 ing to Emmerich, is a combination of the enzyme with some 

 albuminous body. He believes that the bacteriolytic action 

 of normal blood is due to the presence of enzymes. 



ACQUIRED IMMUNITY : This form of immunity is peculiar 

 to the individual, and is extremely variable in its duration. 

 It is acquired either accidentally or experimentally, and is 

 not a hereditary condition. 



Accidental immunity : This usually results from an attack 

 of an infectious disease, like scarlet fever, smallpox, measles, 

 etc. Such an attack confers an immunity which under ordinary 

 conditions is permanent. The immunity may, however, be of 

 only short duration, after which the individual is just as sus- 

 ceptible to the disease as he was before the first attack. In 

 many of the infectious diseases the immunity is of only a few 

 months 7 duration. The attack which confers the immunity 

 need not necessarily be either a severe or a typical one. For 

 instance, a mild attack of smallpox confers as lasting immunity 

 as a severe attack. This is seen in vaccination. 



Experimental immunity : This differs from the accidental 

 form in that it is dependent upon purely artificial conditions. 



