

FIXATION OF THE COMPLEMENT 239 



SECTION V. OPSONINS. 



In studying the bactericidal properties of serums it has been mentioned 

 that many micro-organisms resist, in vitro, the combined action of immune 

 body and complement, but that in the tissues once impregnated with 

 these substances they more readily become the prey of the phagocytes 

 (MetchnikofE). 



Wright and Douglas have shown that in the serum of persons convalescent 

 from infectious diseases or vaccinated against these diseases substances are 

 present which prepare micro-organisms for the action of the phagocytes. 

 Without committing themselves to an expression of opinion as to the nature 

 of these substances Wright and Douglas describe them as opsonins (oi/'wi/ew 

 I prepare). Neufeld has applied to them the name Bacteriotropins. 



According to Wright, opsonins play a fundamental part in the phenomena 

 of immunity : he affirms that it is to opsonins that phagocytosis is due and 

 that by means of the opsonic index of the serum it is possible to measure the 

 immunity of the individual and foresee recovery. 



Metchnikofif has observed that, as a matter of fact, the ingestion of micro-organisms 

 by phagocytes rendered possible by the intervention of opsonins is only one factor 

 in the problem. Ingestion is only of use in so far as it is followed by destruction 

 and digestion of the organisms. But micro-organisms are not destroyed by leucocytes 

 unless the latter contain bactericidal substances or in other words unless the leuco- 

 cytes are " living and strong." Resistant micro-organisms may live for a long 

 time in insufficiently active leucocytes without setting up disease but when such 

 leucocytes are destroyed the micro-organisms are set free and exhibit their powers 

 of producing disease. A notable instance of this is seen in the case of the spores 

 of the tetanus bacillus (Chap. XXXVI.). The opsonic content of the serum is not 

 therefore at any rate in all cases a sufficient datum upon which to evaluate the 

 degree of resistance of the tissues. 



However that may be, opsonins are of sufficient interest in the study of micro- 

 organic diseases and immunizing processes to merit some detailed consideration. 



To determine the opsonic content of a given serum for a particular micro- 

 organism, the serum and a culture of the organism are mixed with normal 

 leucocytes and after an interval the average number of micro-organisms 

 ingested by each leucocyte under these conditions calculated. The number 

 of organisms ingested by 50 leucocytes is counted and the total divided by 

 50 gives the opsonic power of the serum. 



It is obvious, of course, that the number of organisms phagocyted will 

 depend upon the number of bacteria present in a unit volume of the emulsion. 

 The absolute number obtained i.e. the opsonic power is therefore of no 

 value in itself, but if this number be compared with the number which 

 represents the opsonic power of a normal serum determined under the same 

 conditions with the same bacterial emulsion then a standard of comparison 

 is obtained ; and the relation of the opsonic power of the serum of an infected 

 individual to that of a normal individual (measured under identical con- 

 ditions) is known as the opsonic index. 



The amount of opsonin present in the serum of normal individuals is subject 

 to considerable variation and is dependent upon many factors, e.g. the period 

 which has elapsed since food was last taken, pregnancy, etc. (Milhit). 



The amount of specific opsonin in the serum of infected persons shows very 

 curious variation. In tuberculosis, for example, if the opsonic index of normal 

 blood for the tubercle bacillus be taken to be about unity that of infected 

 persons is much lower, and a condition of tuberculosis may be diagnosed in 

 every case in which the opsonic index falls below unity (0'3 to 0'8), provided 

 that the experiment be done several times and the same result is obtained 



