66 
BACTERIOLOGY. 
The 
Widal 
reaction 
length of time, sometimes for life. Instances of rein¬ 
fection are rare. The immunity is conferred by the 
presence in the blood of protective substances known 
as bacteriolysins and agglutinins. The former are 
very much increased after typhoid, and by experiment 
it can be shown that the blood of patients after recov¬ 
ering from typhoid has marked power to dissolve the 
typhoid organisms. The agglutinins possess the power 
of drawing the typhoid bacilli into clusters or clumps. 
This phenomenon is made use of in detecting the pres¬ 
ence of typhoid fever by what is known as the Widal 
reaction. 
It is made in this way: A small amount of blood 
is drawn into a capillary tube from the patient’s ear 
and allowed to clot. By clotting the serum is sep¬ 
arated from the blood. The object of the test is to see 
if the serum contains any agglutinins of typhoid bacilli. 
