190 MANUAL OF BACTERIOLOGY. 



treatment of various diseases. The partial or complete fail- 

 ure of such efforts may be ascribed to the fact that the 

 serum prepared does not contain both the complement and 

 the intermediary or immune body in proper kinds and pro- 

 portions to be effective. There is likely to be a deficiency 

 of complement. Investigators are at present very hopeful 

 of being able to make up for this deficiency. The outlook 

 is encouraging at the present time particularly with regard 

 to typhoid fever and dysentery. 



Welch's Hypothesis. When bacteria do not form sol- 

 uble toxins, we are obliged to suppose that large numbers 

 of them must disintegrate in the body of the host. Thus 

 their intracellular toxins may be liberated and produce the 

 symptoms of intoxication manifested in infections of this 

 character. Welch has also suggested that production of 

 toxins may occur in infection of the living body even when 

 test-tube cultures of the same bacteria contain no soluble 

 toxins or only small amounts. Bacteria growing as para- 

 sites in the living body may adapt themselves to the condi- 

 tions found there. The existence of the bacteria requires 

 them to have some means of combating the substances 

 elaborated by the host for his protection. Bacteria may 

 also be able to form similar substances for their own protec- 

 tion. Such substances, from the standpoint of the host, 

 would constitute toxins. 1 



Nuttall made the important discovery that the serum of 

 the blood deprived of all cells possesses the power of destroy- 

 ing pathogenic bacteria. 



The ingredients of the blood-serum that exert the bacteri- 

 cidal influence have been named alexins, 2 they have also 

 been called defensive proteids. The nature of these sub- 

 stances has not been determined with certainty. Vaughan 



1 Welch, Bulletin Johns Hopkins Hospital, December, 1902. 



2 This word has unfortunately been used partly in another sense, sec 

 page 1 88. 



