102 BACTERIOLOGY 



the relative amount of opsonins in an individual's blood 

 when compared with a normal standard. If the average 

 number of germs contained in a leukocyte in a normal 

 serum were 5, and the average of the individual's blood 

 Under examination but 3, his opsonic index would be 

 said to be 3 divided by 5, or f, or .60. 



If under these circumstances a certain number of dead 

 germs be injected into this individual after a certain 

 length of time, an examination of the blood will show an 

 increased number of bacteria hi each leukocyte. The 

 opsonic index is said to have been raised. 



Like antitoxins, bacterial vaccines are not only of 

 value in the treatment of infections, but they may also 

 be used in the prevention of infection. This is particu- 

 larly true of typhoid vaccine, which confers a marked 

 degree of immunity to typhoid fever. 



Immediately following the administration of a bacteria 

 vaccine there occurs a diminution of the opsonins, which 

 lasts for a variable length of time. This is termed the 

 negative phase. This is followed shortly by the increase 

 of opsonins, or the positive phase. 



It has been contended by some that hi protective 

 vaccination this so-called negative phase lays the sub- 

 ject more liable to infection during this time. Experi- 

 ments, however, have shown that this phase may be 

 practically abolished by using very small initial doses of 

 the vaccine, repeating the vaccinations with larger doses 

 later, that is, in eight to ten days. 



