IMMUNITY 271 



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as described by Metchnikoff and Wright, but in many instances neither of these 

 hypotheses harmonize with the facts. 



The method of attack of different groups of bacteria varies, the results of 

 their assaults vary and the consequent defence and resistance of the host vary 

 to combat these dissimilar infections. 



Thus we observe in diphtheria no invasion by the bacteria but intoxication 

 with extracellular toxin and this is combated largely if not entirely by anti- 

 toxin. 



In typhoid and paratyphoid infections, leucocytosis is conspicuous by its 

 absence; convalescent and immune persons having agglutinins, precipitins and 

 lysins in their blood serum in greater amount than before, in greater amount 

 than is generally observed in the serum of persons not immune to typhoid 

 infection. 



In acute, localized staphylococcus and streptococcus infections, leucocyto- 

 sis and phagocytosis is marked; the convalescent patient's serum contains 

 practically no antitoxin, little if any precipitin, agglutinin or lysin, but does 

 contain an increased amount of opsonin. 



After infection or treatment with dead bacteria or bacterial products the 

 result is not always immunity or exalted resistance. Some species of bacteria 

 may infect one time after time apparently leaving the host.more susceptible 

 to subsequent infection than he was primarily, suggesting that cellular reaction 

 as described by Ehrlich and Metchnikoff does not always occur and that recovery 

 is possible without it. 



ANAPHYLAXIS 



Metchnikoff observed in unicellular organisms that phagocytosis was not 

 confined to invading parasites and food; inanimate poisons frequently being 

 disposed of in the same way. Also in the study of agglutinin, precipitin and 

 lysin production, Ehrlich and his co-workers discovered that such antibodies 

 were generated by the injection of many inanimate proteins. 



Using the term antigen (haptin) to designate a substance which when intro- 

 duced into an animal is a foreign irritant, or splits into simpler compounds some 

 of which are irritant or toxic, and stimulates the cells of the injected animal to 

 produce specific agglutinins, precipitins, antitoxins, lysins, or other antibodies 

 or specific ferments, and as a result makes the injected animal either immune or 

 hypersusceptible to subsequent injections, we find a great variety of proteins 

 in common animal and vegetable matter and in foods, act as antigens. 



As to why the injection of dead typhoid bacilli (antigen) or infection with 

 living typhoid bacilli (antigen) immunizes against .future invasion by typhoid 

 bacilli, and injection or ingestion of red blood cells from another species (anti- 

 gen) or blood serum from another species (antigen) or egg albumin (antigen) 

 creates a hypersensitiveness to the injurious effect of similar subsequent injections 

 of these substances is a matter of speculation. 



This sensitization by foreign proteins is a serious phenomenon of common 

 occurrence. 



When produced by bacteria or bacterial products it is referred to as allergy. 



