PASSIVE IMMUNITY. 435 



the toxine protects also from injections of the living viru- 

 lent bacillus. 



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 infec- 

 tion by the living organism when injected under con- 

 ditions similar to the above. This serum is, therefore, 

 antimicrobic, or preventive against invasion by a particular 

 organism. 



In a considerable number of instances, an antimicrobic 

 serum has been found to possess little effect against the 

 toxine 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 quantity. 



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

 microbic sera have a distinct action on the vital activity of 

 the corresponding bacterium, an action which may, in 

 most instances, be called indirectly bactericidal. It is 

 manifest that such an action differs fundamentally in its 

 nature from the power of protecting against a toxine. It 

 must not be supposed, however, that a serum must be 

 purely antitoxic or purely antimicrobic, according to the 

 method by which it is prepared. For example, an anti- 

 toxic serum can readily be obtained by injecting living 

 diphtheria bacilli into the tissues of an animal, the anti- 

 toxic property being in all probability developed by means 

 of toxines formed by the bacilli within the body. Having 

 given this explanation, we shall describe the modes of 

 preparation of the two kinds of serum and discuss their 

 properties separately. 



i. Antitoxic Serum. The best examples are the anti- 

 toxic sera of diphtheria and tetanus, though similar prin- 

 ciples and methods are involved in the preparation of the 

 sera protective against ricin and abrin, and against snake 

 poison. We shall here speak of diphtheria and tetanus. 

 The steps in the process of preparation may be said to be 

 the following : First, the preparation of a powerful toxine. 



