CELLULAR RESISTANCE 163 



ingham has also shown that the injection of melanin produces a specific 

 opsonic serum and others have shown that carbon granules, cinnabar, 

 carmine, etc., are phagocyted much more readily in the presence of 

 serum than otherwise. In these latter instances it seems probable that 

 the serum provides a protein capsule for the pigment granules, thus 

 facilitating the action of opsonin, but at the present time no satisfactory 

 explanation has been offered for the production of a specific immune 

 opsonin following the injection of melanin. Neufeld and Ungermann 

 point out the difficulty of satisfactory measurement of phagocytic action 

 against pigment granules, and it is possible that this source of error 

 may be sufficient to throw doubt on the results claimed to have been 

 obtained with insoluble pigments. 



Specificity and other Characters of Opsonins. The specificity of 

 the immune opsonins is clear-cut, as has been shown by numerous in- 

 vestigators. An immune opsonin produced by vaccination with staphyl- 

 ococci shows a marked influence on the phagocytosis of the antigenic 

 organisms but none whatever on non-related organisms such as colon 

 bacilli. As in the case of other immune bodies, group reactions are 

 demonstrable. Vaccination with typhoid bacilli leads to the formation 

 of immune opsonins which operate in high degree on the antigenic 

 organism and also to less degree on closely-related organisms such as 

 those of the paratyphoid groups. Dean, in working with serum dilu- 

 tions in order to demonstrate that an optimum concentration of opsonin 

 may not necessarily be found in undiluted serum, reports the following 

 experiment. This may be interpreted as showing an inhibition zone in 

 the stronger concentrations, although the differences are so slight as 

 to fall within the limit of experimental error. 



Dilution of serum Phagocytic index* 



o 9-7 



1-2 9.6 



1-4 10.0 



1-8 8.2 



1-16 8.5 



1-32 64 



* Average number of bacteria ingested per leucocyte. 



Influence of Phagocyte and Ingested Elements. The foregoing 

 paragraphs have considered the influence of serum on phagocytosis, but 

 detailed studies have shown that certain considerations in regard to both 

 the bacteria and the leucocytes exercise some influence. Neufeld pointed 

 out that bacterial cultures from ten to twenty-four hours old are best 

 for in vitro experiments. The reaction takes place best when the bac- 

 teria are suspended in equal-parts broth and physiological salt solution, 

 but in ordinary laboratory practice salt solution is used without the 

 addition of broth and whatever deterring action is exercised by the salt 

 is constant in the series of experiments. The thickness of the sus- 

 pension is of importance since very thin suspensions determine a 

 reduction in phagocytic index as compared with thicker suspensions. 

 The optimal density of the suspensions varies with different bacteria 



