OPSONINS 211 



say in a case of furunculosis, which is almost always caused 

 by the M. pyogenes, the following are required : (1) a 

 drop or so of the patient's serum ; (2) a drop of serum 

 from a normal person ; (3) a suspension in salt solution, 

 of a culture of M. pyogenes preferably derived from the 

 furuncle ; (4) leucocytes washed free from the plasma. 

 Equal volumes of the patient's serum, leucocytes, and 

 suspension are mixed, draw^n up in a capillary tube, incu- 

 bated for fifteen minutes at 37 C., and films are then 

 prepared and stained. As a control a similar mixture is 

 prepared and treated in the same way, but using the normal 

 serum instead of that of the patient. The films are then 

 examined, and the number of cocci taken up by, say, 

 fifty leucocytes is counted in the two specimens, and a 

 ratio obtained. Taking the figure for the normal serum 

 as 1, that for the patient's serum will probably be 0-5 or 

 0-6, and this is termed the " opsonic index " (see below, 

 p. 219). 



In subacute and chronic local infections the opsonic 

 value of the serum is usually diminished, occasionally 

 increased. In acute infections the index will, as a rule, 

 below ; in chronic infections which are not strictly localised, 

 e.g. tuberculosis, the index will sometimes be low, some- 

 times high. A low index generally indicates an infection, 

 or a low power of resistance to the particular organism, 

 or that a chronic but quiescent infection exists ; a high 

 index may indicate that the person has had an infection 

 but has overcome it, or has a quiescent infection. The 

 normal index for healthy persons varies only within 

 narrow limits, from about 0-8 to 1-2 as extremes ; an index 

 above or below these values is therefore probably patho- 

 logical. 



By injecting small quantities of a vaccine consisting of 

 a killed culture, tuberculin, etc., the opsonic index can 

 be raised, and the infection thereby tends to be cured. 



