IMMUNITY AND ANAPHYLAXIS 183 



also could defibrinated blood, the pericardial fluid and 

 the aqueous humour, and that this power was lost by 

 heating to 55 C. Buchner (with others) found that 

 completely cell-free blood was bactericidal, and lost this 

 power on heating to 55 C, but not on freezing and 

 thawing. According to Buchner, fresh blood frozen and 

 thawed loses its power because the red cells are destroyed 

 by the process, and make the blood so suitable for 

 bacteria that the bactericidal power is compensated for. 

 These protective substances in the serum are called 

 " Cytases " by Metchnikoff, and "Alexines " by Buchner. 

 These are now believed to be derived from the leucocytes, 

 Metchnikoff holding that they are only formed on the 

 death of the leucocytes, or rather phagocytes (" phago- 

 lysis"), and that they do not exist in the body except 

 under abnormal conditions. In any case when present they 

 will probably prepare the bacteria for ingestion by the 

 phagocytes, and thus are related to, if not identical with 

 the Opsonins " (feast-preparers). The cytases or alex- 

 ines are of proteid nature and are very unstable. The 

 withdrawal of the salts from the serum by dialysis sus- 

 pends their activity, which is restored on again adding 

 them. This fact is evidently related to the lessened 

 amount of chlorides excreted in the urine in all acute 

 febrile processes, and especially in lobar pneumonia. 

 The cytases or alexines may be regarded as an appliance 

 common to every animal organism, for the dissolution of 

 organized substances, whether bacteria, foreign red 

 corpuscles, or other foreign bodies. They are the 

 " Complements" of Ehrlich's classification. 



Acquired Immunity. The immunity that is called 

 natural is of a general kind, being a natural resistance to 

 disease or bacteria of all kinds. That type of immunity 

 called acquired, is, on the other hand, "specific" in kind, 

 that is, an immunity from a definite or specific disease or 

 infection. For this reason it is by some called specific 

 immunity. 



(a). Acquired by an attack of a Specific Disease. The 

 fact that an attack of small-pox followed by recovery 

 protected the individual from further attack, was a notable 

 one during the epidemic prevalence of that disease. Similar 



