222 THE MECHANISM OF IMMUNITY 



immunity so acquired is lasting and occasionally absolute. By increasing the 

 number of successive vaccinating inoculations the animal may in time become 

 so highly immunized that even enormous doses of the specific organism or 

 toxin have no visible effect upon it : this is a special condition of hyper- 

 immunization in which the resistance of the animal is raised to its highest 

 limits. 



But if a non-immune subject be inoculated with the serum of an immunized 

 or hyper-immunized animal instead of with organisms or toxins a different 

 result ensues. The former is certainly rendered immune but in this case it 

 is merely a condition of passive immunity. The person or animal passively 

 immunized has taken no active part in the process of immunization but has 

 simply been inoculated with something possessing prophylactic properties. 

 The period during which such immunity lasts, which is always very short 

 (generally a few days only), is dependent upon the time during which the 

 substance inoculated remains in the tissues and as soon as it is eliminated 

 the immunity has gone. 



The mechanism of immunity. 



If a living animal be immune against a pathogenic organism, the inocula- 

 tion of that organism into the animal results in an aggregation of leucocytes at 

 the site of inoculation (chemiotaxis) which ingest and digest the inoculated 

 organisms. This is the phenomenon described by Metchnikoff as phago- 

 cytosis. 



Phagocytosis can be easily observed, for instance, with the anthrax bacillus. 

 If a healthy guinea-pig be inoculated with a trace of an anthrax culture the 

 tissues about the site of inoculation soon become the seat of an cedematous 

 infiltration (the oedema consists of a serous fluid containing free organisms 

 but very few leucocytes) : the bacillus quickly generalizes and death rapidly 

 supervenes. On the other hand, if a guinea-pig previously vaccinated 

 against anthrax be inoculated it can be shown that numbers of leucocytes 

 very rapidly accumulate at the site of inoculation and in a few hours have 

 ingested, killed and digested all the bacilli, the animal suffering no ill-effects 

 from the inoculation. A similar observation can be made on dogs, animals 

 naturally immune to anthrax. The inoculation of anthrax bacilli into dogs 

 is followed by a small abscess in which phagocytosis is very active but the 

 infection does not become generalized. 1 



The leucocytes take up the micro-organisms while the latter are still living. 

 Experiments have been devised to show that organisms ingested by leucocytes 

 retain their vitality for a greater or lesser length of time during which they 

 can, in a non-immune animal, set up a fatal infection (Metchnikoff). 



On the other hand, in some cases, notably in the case of the cholera vibrio, 

 it has been observed that if the vibrio be inoculated into the peritoneal cavity 

 of an immunized guinea-pig it is killed not after ingestion by the leucocytes 

 which are present in very small numbers in the exudate but in the exudate 

 itself : this constitutes Pfeiffer's phenomenon (vide infra). Such a phenomenon 

 might be quoted as an objection to the theory of phagocytosis but more 

 extended observation shows bactericidal action of this nature by the body 

 fluids to be exceptional : it may be described as a make-shift in the defence 

 of the individual and only occurs when the leucocytes have undergone changes 

 which prevent them coming in contact with the organisms themselves and 

 is moreover only seen in the case of a few very delicate organisms. 



1 Micro-organisms have their own means of defence in their fight with the leucocytes : 

 they secrete soluble substances, agressins, which act on the white cells of the blood and 

 prevent them ingesting and destroying the infecting agents. In conditions of immunity 

 the leucocytes triumph over these agressins and thus fulfil their function of defence. 



