PASSIVE IMMUNITY. 469 



B. Passive Immunity. 



Action of the Serum of Highly-Immunised Animals. 

 i. The serum of an animal A, treated by repeated and 

 gradually increased doses of the toxine of a particular 

 microbe, may protect an animal B against a certain amount 

 of the same toxine 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 protective or 

 palliative power. Seeing that the serum of animal A 

 appears to neutralise the toxine, the term antitoxic has been 

 applied to it. It also protects under like conditions from 

 infection with the corresponding microbe. Thus an anti- 

 diphtheritic serum prepared by injections of the toxine 

 protects also from injections of the living virulent 

 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 conditions 

 similar to the above. This serum is therefore antimicrobic, 

 or preventive 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 

 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 an antimicrobic serum has 

 a distinct action on the vital activity of the corresponding 

 bacterium, and may even produce alteration in its structure. 

 It is manifest that such a serum differs fundamentally in 

 its point of attack, so to speak, from an antitoxic serum. 

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



