182 CLINICAL BACTERIOLOGY AND 



place in animals. The modus operandi advised by Vallee 

 is as follows : 



The hair is clipped and the skin shaved over the shoulder- 

 blade about the size of a five-shilling piece, and the cutis 

 scarified with a sterile lancet and a 50 per cent, or concen- 

 trated solution of tuberculin painted on with a brush. 



If the reaction is a positive one, a visible infiltration of 

 the border of the wound, and an oedematous infiltration of 

 the area surrounding the seat of operation, follows, usually 

 reaching its maximum on the second day. In a negative 

 case there is only a slight indication of irritation, due to 

 the direct effect of the lancet. As it is advisable to have 

 a control area, a similar surface of skin should be scarified 

 on the opposite side, and not treated with tuberculin. 



Many theories have been put forward with reference to 

 the tuberculin reaction ; the processes which bring it about 

 are probably most satisfactorily based upon what is known 

 as " anaphylaxis." 



It has been found that injections of many substances 

 other than bacteria and their products cause reactions on 

 the part of the tissues, with consequent production of 

 antibodies. These substances are called "antigens"; an 

 antigen is therefore a tissue stimulant, and the product of 

 the stimulation is an antibody. 



Tuberculin may be injected into a healthy animal, and 

 produce no effect whatever ; if injected into a tubercular 

 patient, a reaction follows. There must be some great 

 difference in the state of two such animals, and the conse- 

 quent reaction in the diseased animal is due to the probable 

 fact that it has become sensitized against the bacterial 

 constituents. 



This may also explain why one injection of tuberculin 

 prohibits a reaction to a second injection up to a given 

 lapse of time, the body being in a state of anti-anaphylaxis, 

 the sensitizing elements having become exhausted pro tern., 

 and requiring time for their restoration. 



When the tuberculin fails to cause a reaction in these 



