ADJUSTMENT TO INFECTIOUS DISEASE 425 



substance the molecular equilibrium of the bacterial sus- 

 pension is altered, so that precipitation occurs, just as 

 many colloidal suspensions may be precipitated in the 

 presence of electrolytes. If we allow bacteria to absorb 

 their particular antibody out of a serum and wash this 

 united complex in distilled water, these so-called "sensitized" 

 bacteria will remain unprecipitated; but add an electrolyte 

 by resuspending them in salt solution and rapid precipitation 

 or agglutination will occur. 



Furthermore, by the absorption of the antibodies the 

 bacteria become vulnerable to two effective influences in 

 the body. One is an enzyme-like constituent in the circulat- 

 ing blood which is easily destroyed by heat and deteriorates 

 on standing, but which is always present in fresh blood. 

 It is called by immunologists "alexin" or "complement." 

 This active serum constituent of normal animals exerts a 

 destructive, sometimes even a solvent eff"ect upon bacteria, 

 relatively slight in the case of normal bacteria, but materi- 

 ally enhanced after the bacteria have in some manner been 

 changed by the union with their antibodies. 



Again, there are in the bodies of all animals definite cells 

 usually spoken of as the "white blood cells" and certain 

 wandering cells of the reticulo-endothelial system (clas- 

 matocytes and phagocytic endothelial cells of various kinds) 

 which have retained the primitive capacity for intracellular 

 digestion. These take up foreign particles that gain access 

 to the body. The particular type of these cells that we are 

 capable of studying in the test tube, namely, the leucocytes of 

 the blood, will not take up bacteria to any extent if washed 

 leucocytes are brought together with bacteria in physiological 

 salt solution. In the presence of normal serum they can 

 take up many bacteria of the less virulent varieties, but will 

 often entirely fail to ingest bacteria of very virulent strains, 

 like pneumococci or virulent anthrax bacilli. After such 

 bacteria, however, have united with antibody, they are so 

 altered that the phagocytes can take them up and destroy 

 them actively and in large numbers. 



The mechanism by which the union with specific antibody 

 modifies bacteria is still, to a great extent, obscure. By 

 the absorption of antibody there is a reduction of electrical 



