258 A MANUAL OF BACTERIOLOGY 



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 pathological. 



By injecting small quantities of a vaccine consisting of 

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

 usually be raised, and coincidently the infection tends to 

 be cured. The first effect of the injection is to cause a fall 

 in the opsonic index, the " negative phase " of Wright, 

 which is usually afterwards followed by a rise, and by 

 properly spacing the injections a considerable rise in the 

 opsonic value may ultimately result. If doses be given 

 too close together, successive negative phases may be 

 superimposed, and the index becomes unduly depressed, 

 it may be for a long period. Similarly, if too large a single 

 dose of vaccine be given the effect may be to depress the 

 index for a long period and cause harm instead of good. 

 It is better, therefore, to commence treatment with a 

 small dose, to allow an interval of about a week between 

 the doses, and to increase the dose's gradually (for dosage, 

 etc., see p. 265). By movement, massage, etc., applied 

 at or about the seat of a local infection, bacterial products 

 are disseminated and may alter the index ; a process of 

 auto -inoculation may thus result. 



The opsonic index may be used for diagnostic purposes ; 

 a low or high opsonic value towards a particular organism 

 suggests that an infection by this organism exists or has 

 recently existed. 



The nature of opsonin has given rise to much discus- 

 sion. As we have seen, opsonin is present in normal 

 serum and is thermolabile, being destroyed at 60 C. 

 The normal opsonin seems to be partly " common " and 

 partly specific. The immune opsonin, on the other hand, 



