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Materials for skin testing
Editorial MATERIALS
FOR SKIN TESTING
In the minds of the public and the great majority of the medical profession, an allergist is a doctor who does skin tests. Much as the allergist may seek to broaden his diagnositic approach and exercise clinical judgment, his main distinction from the remainder of his profession is in his ability to discover the specific causesof allergic diseaseby demonstration of positive skin reactions, and his usefulness is judged to a great degree on the accuracy of his testing methods. An etiologic diagnosis backed by a positive skin test appeals to both patient and family doctor, while a diagnosis made on clinical grounds in the face of a negative test may be considered only a shrewd guess, to be accepted or rejected depending upon whether or not it agrees with one’s own opinion. It therefore behooves the practitioners of allergy to examine critically the methods used in performing skin tests in an effort to obtain the greatest possible correlation between skin reactions and clinical sensitization. In general, skin tests with pollens and inhalants, when properly performed and interpreted, yield sufficiently reliable results to be of great value in diagnosis. On the other hand, skin tests with foods give so many reactions, both “false positives” and “false negatives” which do not agree with the clinical symptoms, that many physicians, both allergists and others, have come to the belief that they are of little or no value. Some of these inconsistencies are explainable. A positive skin reaction to a food which the patient can eat with impunity may mean that the antigenic properties of the original food have been so changed in cooking, digestion, adsorption, and metabolism that no significant amount of active antigen reaches the shock organ through the circulation. Some delayed clinical allergic reactions to foods associated with negative skin tests have been shown by Cooke1to result from sensitization, not to the unaltered food, but to a proteose or other product of its digestion. In such cases,skin tests with suitable preparations of the artificially digested food give positive reactions. However, evidence is gradually accumulating that failure to demonstrate positive skin reactions in certain clear-cut casesof clinical food allergy is not due to changes in the antigens as a result of digestion and metabolism, but simply to failure to employ the antigens in an active form for skin tests. Many an allergist has been embarrassedby his inability to demonstrate positive skin reactions in patients who know from repeated experience that they develop allergic skin rashes after eating berries or shellfish. Tuft and Blumstein2 in 1942 showed that the antigens of berries and certain other fruits were very easily destroyed in the preparation of extracts for skin testing. In patients known to be sensitive to these foods, in whom skin tests with the available extracts gave no reaction, the allergy was readily demonstrated by applying a drop of fresh juice to a scratch. More recently, Ancona and Schumacher3 have demonstrated similar results in patients allergic to shellfish. In 14 patients clinically sensitive to 379
380
THE
JOURNBL
OF
ALLERGY
shrimp, 3 different commercial extracts ga.ve little or no reaction, but fresh 01 quick-frozen material gave positive reactions in scratch tests. Evidence has been offered that these reactions are specific. Preliminary studies indicate that a similar situation may apply to other groups of foods, as well as fruits and shellfish. This superiority of a relatively primitive method of testing reflects our ignorance of the nature of antigenic materials in many foods. Presumably the fruits, berries, and shellfish contain antigens which are readily destroyed by enzymatic action or chemical manifestations, but are preserved in a stable form by quick freezing. Even antigens which remain active after cooking may be lost One patient who was clinically sensitive to in the preparation of an extract. peaches gave a marked reaction to a scratch test with the juice of home-canned peaches; the same juice after ext,raction and Seitz filtration gave no reaction on intracutaneous test. Such results pose difficult problems, but also offer promise of substantial improvement in the results of food tests. To meet the demand for allergenic extracts on a wide variety of foods, the methods found suitable for extracting pollens have been applied, with such simple modifications as neutralization of excess acidity. The studies of Tuft and Blumstein and Ancona and Schumacher indicate that these methods are not applicable to many foods. Preparation of reliable allergenic extracts of these foods requires development of new methods, based on further knowledge of the properties of the antigens they contain. This field offers interesting possibilities for research. Pending a solution of this problem, Ancona and Schumacher have suggested the routine use of quickfrozen foods for scratch tests, and have presented a practical method of handling them. Further studies of the comparative results obtained by these two methods in patients clinically sensitive to all types of foods are needed to establish which extracts are efficient and which are not. W. B. S. REFERENCES
1. Cooke, R. A.: Protein Derivatives as Factors in Allergy, Ann. Int. Med. 16: 71, 1942. 2. Tuft, L., and Blumstein, G. I.: Studies in Food Allergy, J. ALLERGY 13: 574, 1942; 15: 346, 1944. The Use of Raw Foods As Testing Material in 3. Ancona, G. R., and Schumacher, I. C.: Allergic Disorders, California Med. 73: 473, 1950.
FOR SKIN TESTING
In the minds of the public and the great majority of the medical profession, an allergist is a doctor who does skin tests. Much as the allergist may seek to broaden his diagnositic approach and exercise clinical judgment, his main distinction from the remainder of his profession is in his ability to discover the specific causesof allergic diseaseby demonstration of positive skin reactions, and his usefulness is judged to a great degree on the accuracy of his testing methods. An etiologic diagnosis backed by a positive skin test appeals to both patient and family doctor, while a diagnosis made on clinical grounds in the face of a negative test may be considered only a shrewd guess, to be accepted or rejected depending upon whether or not it agrees with one’s own opinion. It therefore behooves the practitioners of allergy to examine critically the methods used in performing skin tests in an effort to obtain the greatest possible correlation between skin reactions and clinical sensitization. In general, skin tests with pollens and inhalants, when properly performed and interpreted, yield sufficiently reliable results to be of great value in diagnosis. On the other hand, skin tests with foods give so many reactions, both “false positives” and “false negatives” which do not agree with the clinical symptoms, that many physicians, both allergists and others, have come to the belief that they are of little or no value. Some of these inconsistencies are explainable. A positive skin reaction to a food which the patient can eat with impunity may mean that the antigenic properties of the original food have been so changed in cooking, digestion, adsorption, and metabolism that no significant amount of active antigen reaches the shock organ through the circulation. Some delayed clinical allergic reactions to foods associated with negative skin tests have been shown by Cooke1to result from sensitization, not to the unaltered food, but to a proteose or other product of its digestion. In such cases,skin tests with suitable preparations of the artificially digested food give positive reactions. However, evidence is gradually accumulating that failure to demonstrate positive skin reactions in certain clear-cut casesof clinical food allergy is not due to changes in the antigens as a result of digestion and metabolism, but simply to failure to employ the antigens in an active form for skin tests. Many an allergist has been embarrassedby his inability to demonstrate positive skin reactions in patients who know from repeated experience that they develop allergic skin rashes after eating berries or shellfish. Tuft and Blumstein2 in 1942 showed that the antigens of berries and certain other fruits were very easily destroyed in the preparation of extracts for skin testing. In patients known to be sensitive to these foods, in whom skin tests with the available extracts gave no reaction, the allergy was readily demonstrated by applying a drop of fresh juice to a scratch. More recently, Ancona and Schumacher3 have demonstrated similar results in patients allergic to shellfish. In 14 patients clinically sensitive to 379
380
THE
JOURNBL
OF
ALLERGY
shrimp, 3 different commercial extracts ga.ve little or no reaction, but fresh 01 quick-frozen material gave positive reactions in scratch tests. Evidence has been offered that these reactions are specific. Preliminary studies indicate that a similar situation may apply to other groups of foods, as well as fruits and shellfish. This superiority of a relatively primitive method of testing reflects our ignorance of the nature of antigenic materials in many foods. Presumably the fruits, berries, and shellfish contain antigens which are readily destroyed by enzymatic action or chemical manifestations, but are preserved in a stable form by quick freezing. Even antigens which remain active after cooking may be lost One patient who was clinically sensitive to in the preparation of an extract. peaches gave a marked reaction to a scratch test with the juice of home-canned peaches; the same juice after ext,raction and Seitz filtration gave no reaction on intracutaneous test. Such results pose difficult problems, but also offer promise of substantial improvement in the results of food tests. To meet the demand for allergenic extracts on a wide variety of foods, the methods found suitable for extracting pollens have been applied, with such simple modifications as neutralization of excess acidity. The studies of Tuft and Blumstein and Ancona and Schumacher indicate that these methods are not applicable to many foods. Preparation of reliable allergenic extracts of these foods requires development of new methods, based on further knowledge of the properties of the antigens they contain. This field offers interesting possibilities for research. Pending a solution of this problem, Ancona and Schumacher have suggested the routine use of quickfrozen foods for scratch tests, and have presented a practical method of handling them. Further studies of the comparative results obtained by these two methods in patients clinically sensitive to all types of foods are needed to establish which extracts are efficient and which are not. W. B. S. REFERENCES
1. Cooke, R. A.: Protein Derivatives as Factors in Allergy, Ann. Int. Med. 16: 71, 1942. 2. Tuft, L., and Blumstein, G. I.: Studies in Food Allergy, J. ALLERGY 13: 574, 1942; 15: 346, 1944. The Use of Raw Foods As Testing Material in 3. Ancona, G. R., and Schumacher, I. C.: Allergic Disorders, California Med. 73: 473, 1950.