PHAGOCYTOSIS 283 



which can prepare bacteria for phagocytosis. There is a thermo- 

 labile substance which occurs in normal serum, and a thermo- 

 stable one which is found in immune serum ; and this latter also 

 contains a thermolabile substance, since (as a rule) its index is 

 lowered by heat. Thermostable opsonin occurs in minute traces 

 in normal serum, since the index is never reduced quite to the 

 level seen in a control specimen made with normal saline by 

 heating to 60 C , and we need have no hesitation in recognizing it 

 as a specific antibody. It will be convenient to deal with it first, 

 and the question naturally arises, Is it amboceptor ? In other 

 words, Has amboceptor the power of preparing bacteria for 

 phagocytosis in addition to sensitizing them to the action of 

 complement ? The two substances arise under the same con- 

 ditions, and are identical in their power of resisting heat, faculty 

 of combining with bacteria, and in their specificity. The second 

 question arises, Assuming thermostable opsonin is amboceptor, is 

 the action of complement also useful in preparing bacteria for 

 phagocytosis, or does the process go on equally well without it ? 

 Now it is certain that complement is not necessary for the action 

 of thermostable opsonin; otherwise it would only exert its action 

 in a heated serum when subsequently activated by fresh serum, 

 and this is not the case. If thermostable opsonin is amboceptor, 

 therefore, it can exert its effects without the action of com- 

 plement. But some experiments go to show that thermostable 

 opsonin may be more potent when reactivated. Thus Crofton 

 found an antistreptococcic serum might stimulate phagocytosis 

 more when mixed with fresh human serum than with an equal 

 amount of normal saline. 



Similar results have been obtained more recently by Dean, 

 who finds that the opsonic effect obtained by heated serum and 

 normal serum may be greater than the sum of the two effects 

 separately. The subject has been very carefully investigated by 

 Chapin and Cowie, who were able to avoid the possibility of 

 certain errors by performing their saturation experiments in a 

 cold room, kept at o C. throughout the experiment. They found 

 that a normal human serum treated with staphylococci at this 

 temperature might have the whole of its opsonic power removed, 

 and yet would still reactivate a heated serum i.e., the thermo- 

 stable opsonin combines with bacteria at o C., and is probably 

 amboceptor. They found that staphylococci treated with normal 

 serum at o C. and then washed are slightly more susceptible to 



