While the bactericidal power of human sera over B. typhosus is increased in a proportion of persons following typhoid fever or active immunization with a vaccine, there is no direct relation between the typhoidin skin reaction and the results of bactericidal tests in vitro.
Agglutinins or complement-fixing antibodies or both are present in the blood serum of the majority of persons reacting positively in the skin test and particularly among those actively immunized, but there is no definite relation between these as either may be in evidence in the absence of the other.
Powdered typhoidin and its control produces severe reactions when injected intracutaneously in doses of 0.0005 to 0.001 mgm.; these reactions and particularly that produced by the control, render the reading and interpretation of the test quite difficult and subject to much error.
Cutaneous anaphylaxis to typhoidin was found apparently to persist for a longer time among those who have had typhoid fever than among those actively immunized with the vaccine. Among the latter the highest percentage of reactions was found during the first year following immunization.
While the typhoidin reaction indicates sensitization to typhoid protein, there is not yet sufficient evidence to warrant its acceptance as an index of immunity in typhoid fever.