120 IMMUNOLOGY 



sirable, although for convenience shorter intervals are often used. 

 After an immunity is once established, the question of how soon 

 is it necessary to revaecinate arises. 



In the case of smallpox, the Kansas Citj^ epidemic showed that no 

 one successfully vaccinated within six months of exposure con- 

 tracted the disease. Ordinarily it would seem wise to revaecinate 

 whenever exposure occurs. If one is immune, the onlj- reaction will 

 be the immune reaction described by Jenner and this is not ob- 

 jectionable. 



In the case of typhoid immunization, many advise revaccination 

 every 2 years. With whooping cough and tetanus it is possible 

 that immunity will last for several years although frequently 

 revaccination may be indicated when exposure occurs. The time 

 for revaccination against diphtheria can be determined by means 

 of the Schick test. 



An interesting phenomenon that may be of importance is the 

 anmnnestic reaction which is the basis for the timing of the sec- 

 ondary stimulus in vaccination. While it takes days or weeks for 

 antibodies to develop following the one or more doses constituting 

 the primary stimulus, after immunity is established, a secondary 

 stimulus or injection of vaccine will lead to a rapid formation of 

 antibody. It has been noted that in children whose Schick test 

 was negative following immunization and a few years later be- 

 came positive the amount of toxin used in the positive Schick 

 test caused a rapid rise in their antitoxin titer so that in retesting 

 they were negative. This is an example of the anamnestic reaction. 

 It is because of this ''hair-trigger" mechanism (anamnestic re- 

 action) that revaccination following exposure is substituted. The 

 revaccination dose is a secondary stimulus and causes a rapid rise 

 in antibody titer. It is well established that the secondary stimulus 

 does not have to be a large dose of antigen. 



It should be remembered that, granting a good antigen and cor- 

 rect method of administering a vaccine, some individuals do not 

 develop immunity. Furthermore, killed bacterial antigens (e.g., 

 E. typhosa and H. pertussis) do not stimulate as great an im- 

 munity as results from the disease. 



An explanation of this is, in part at least, to be found in the 

 discoveries of Felix and Pitt (1934) and others studying the 



