376 BIOLOGY: GENERAL AND MEDICAL 



This kind of treatment, known as immunization, is 

 entirely artificial and it is doubtful' whether anything 

 like it can occur in nature. It leads, however, to a 

 physiologico-chemical change in the experiment animal 

 for its blood is now found to contain a new substance 

 by virtue of which the poisonous power of the venom is 

 neutralized so that if some of the venom and some of 

 the blood serum be mixed together and injected into a 

 new animal, no effect is noticed if the mixture be made 

 in proper proportions. Not only is this true with regard 

 to one fatal dose, but for multiples of that dose, so that 

 twice the fatal dose, with twice the necessary quantity 

 of the neutralizing serum, or five times the fatal dose, 

 with five times the quantity of the neutralizing serum, 

 or perhaps even one hundred or one thousand fatal 

 doses with one hundred or one thousand times the 

 quantity of the neutralizing serum may be administered 

 without injury. Thus as the result of the reactions 

 wrought by the venom, an antidote or neutralizing sub- 

 stance has been produced. 



Substances capable of effecting reactions attended by 

 such results are known as antigens, the substances result- 

 ing from the reactions as anti-bodies. 



Antigens embrace a great variety of substances, many 

 of which appear quite inert or harmless, as white of 

 egg, milk, peptone etc., yet all of which are capable 

 under certain circumstances of bringing about systemic 

 alterations, some of which must be profound. 



Thus, the blood serum of the horse when injected into 

 guinea-pigs is harmless. An ordinary adult guinea-pig 

 may safely be injected with 10 c.c. or even 20 c.c. with- 

 out danger to life. Or guinea-pigs may be given small 

 doses 1 c.c. every few days for an indefinite period 

 without harm; but if the manipulation be modified, most 

 unexpected and extraordinary results may follow. 

 Thus, suppose the guinea-pig be injected, into the peri- 

 toneal cavity, with 1/250 c.c. of the horse serum and 

 then neglected for about two weeks. No effect can be 



