CHAPTER VII. 
ANAPHYLAXIS, ALLERGY OR HYPERSENSITIVENESS.^ 
Protein fed to man or animals is reduced to simple compounds, 
chiefly amino-acids, by the action of gastro-intestinal enzymes before 
it is absorbed from the alimentary canal. These gastro-intestinal 
enzymes act rapidly under normal conditions, and without an appreci- 
able latent period. One noteworthy result of this digestion is a 
complete denaturization of all ingested protein before it enters the 
tissues of the host; absorption of the unaltered or partially-digested 
protein is prevented or reduced to a minimum. It is necessary at 
this point to emphasize the difference between the naturally occurring 
intestinal proteolytic enzymes— as pepsin, trypsin and erepsin, which 
act upon any protein— and those developed parenterally in the body 
fluids in response to the presence of alien protein, the proteolysins 
(cytolysins, bacteriolysins and hemolysins), which act upon the 
homologous protein solely. Also, a latent period is required for the 
maturation of the specific lysin. 
The importance of a denaturization of protein before it enters the 
tissues becomes apparent when a comparison is made between the 
effects of parenteral injections of the end-products of the digestion 
of a specific protein, on the one hand, and of the unaltered protein 
itself, on the other hand. A single injection, or repeated parenteral 
injections of amino-acids in moderate amounts, appears to be without 
serious or noteworthy effects upon experimental animals. A single 
parenteral injection of an unaltered protein is also without visible 
effect, as a rule. A second parenteral injection of the same protein, 
after an interval of ten to fourteen days, is frequently followed by a 
rather definite train of symptoms, severe in character and wholly 
unlike the negative response to a corresponding treatment with amino- 
acids or normal end-products of gastro-intestinal digestion. The 
phenomena attending the injection of protein and the symptomatology 
following the reinjection are termed anaphylaxis, allergy or hypersen- 
sitireness. For convenience in presentation, sensitization, reinjection 
of the homologous protein and the nature of the reaction are discussed 
in seriatim below. 
Sensitization.— The first parenteral injection of a protein- which is 
foreign to the body, or in some instances, natural for the body but 
1 For an excellent resume of the literature of anaphylaxis, quite complete to 1912, see 
Hektoen: Jour. Am. Med. Assn., 1912, 58, 1081; also Wells: Physiological Reviews, 
1921, 1, 1. 
2 Proteins deficient in histidine, tryptophane or tyrosine are said not to sensitize. 
