ANTIBODIES TO PNEUMOCOCCUS 397 



cytosis by two separate antibodies acting in conjunction with 

 complement. One of these is the type-specific anticarbohydrate 

 antibody reacting with the carbohydrate fraction of Pneumococ- 

 cus ; the other is probably also a type-specific antibody, but quite 

 distinct from the former and, therefore, reacting with a different 

 antigenic constituent of the bacterium. In normal human serum 

 heated to 56° these two antibodies may, after prolonged contact 

 with the organism, promote phagocytosis of Pneumococcus with- 

 out the adjuvant action of complement. The two antibodies are 

 equally effective in the phagocytosis of twenty-four-hour cultures 

 by normal blood, but the anticarbohydrate antibody tends to pre- 

 dominate as the pneumococci approach the state in which they 

 exist in the animal body. The anticarbohydrate antibody was the 

 only one in immune serum which could be demonstrated to induce 

 phagocytosis. It was active by itself, but complement enhanced its 

 effect. 



To Ward and Enders it seemed that a single well-defined anti- 

 body — the anticarbohydrate antibody — might be responsible for 

 the phagocytic action of unheated normal serum, of heated normal 

 serum, inactivated immune serum, and immune serum activated by 

 complement. The facts appeared to invalidate the division pro- 

 posed by Neufeld of the phagocytic antibodies into bacteriotro- 

 pins — antibodies, the phagocytic titer of which is not raised by 

 the addition of complement — and opsonic antibodies — antibodies, 

 comparable to lysins, which are only active in the presence of 

 complement. Complement alone was found to be incapable of in- 

 ducing phagocytosis of Pneumococcus and, therefore, may act 

 merely as a catalyst in increasing the velocity of the phagocytic 

 process. On the basis of their observations, Ward and Enders pro- 

 posed that the term "tropin" be discarded, since it was misleading 

 and unnecessary, and that the term "opsonin" be retained to de- 

 note any heat-stable antibody which prepares bacteria for phago- 

 cytosis. Contrary to current usage the latter term would not sug- 

 gest a combination of antibody with complement. 



