PASSIVE IMMUNITY. 469 



much less certain in results, and so much more tedious than the 

 others, that it has obtained no practical applications. 



Active immunity of -high degree developed by the methods 

 described may be regarded as specific, that is, is exerted only 

 toward the organism or toxin by means of which it has been 

 produced. A certain degree of immunity, or rather of increased 

 general resistance of parts of the body (for example the peri- 

 toneum), can, however, be produced by the injection of various 

 substances — bouillon, blood serum, solution of nuclein, etc. 

 (Issaeff). Also increased resistance to one organism can be 

 thus produced by injections of another organism. Immunity of 

 this kind, however, never reaches a high degree. 



B. Passive Immunity. 



Action of the Serum of highly Immunised Animals. — i . The 



serum of an animal A, treated by repeated and gradually in- 

 creased doses of the toxin of a particular microbe, may protect 

 an animal B against a certain amount of the same toxin when 

 injected along with the latter, or a short time before it. As 

 would be expected, it has less effect when injected some time 

 afterwards, but even then within certain limits it has a degree of 

 curative or palliative power. Seeing that the serum of animal 

 A appears to neutralise the toxin, the term antitoxic has been 

 applied to it. 



2. The serum of an animal A, highly immunised against a 

 microbe by repeated and gradually increasing doses of the living 

 organism, protects an animal B against an infection by the living 

 organism when injected under conditions similar to the above. 

 This serum is therefore antimicrobic, or antibacterial, or pre- 

 ventive against invasion by a particular organism. When spoken 

 of in relation to the bacterium by means of which it has been 

 prepared, a serum is usually called homologous ; in relation to 

 any other bacterium, heterologous. 



In a considerable number of instances, an antimicrobic serum 

 has been found to possess little effect against the toxin — that 

 is, to possess little or no antitoxic power. This fact, if taken 

 alone, would leave it still doubtful whether the difference between 

 the two kinds of sera were one of quality or one merely of quan- 

 tity. It has, however, been shown in many cases that an anti- 

 microbic serum has a distinct action on the vital activity of the 



