1 62 IMMUNE SERA 



and therefore is often ineffective. Consisting, as 

 it usually does, in the injection of an alien serum, 

 passive immunization produces an immunity of but 

 short duration, the body apparently getting rid of 

 the alien proteid as rapidly as possible. The great 

 advantage of this form of immunization, however, 

 is its convenience, freedom from risk to the patient, 

 and above all, the fact that the immunity is pro- 

 duced instantaneously. 



Mechanism of Immunity. Infection, whether 

 natural or artificial, is usually followed by a 

 remarkable series of alterations in the tissues of 

 the infected host. Representing, as it does, all the 

 tissues of the body, it is natural that these changes 

 are most strikingly exhibited in the blood. The 

 alterations vary, however, both with the kind 

 of bacterium, and with the animal species involved. 

 Against the true toxins, including probably the 

 leucocidins and hasmolysins, the body produces anti- 

 toxins; against the bacterial bodies it directs the 

 action of the leucocytes and the lytic combinations 

 formed by the union of amboceptor and comple- 

 ment; against the so-called aggressins it directs 

 the opsonins and perhaps also the bacteriolysins. 



Before leaving the consideration of the reaction 

 of the body to infection, attention should be called 

 to the comprehensive investigations of Opie. This 

 observer showed that the cells which accumulate in 

 response to an irritant contain enzymes, the enzyme 

 of the polynuclear leucocytes resembling trypsin 



