Skin sensitivity had been mentioned and demonstrated in isolated instances over a long period of years before Schloss in 1912 extended and described its usage and established it as a definite means of detecting hypersensitiveness. Since then it has been universally recognized as a most useful adjunct to the study of the aetiological problems of allergy. At first it seemed that a miraculous means had been found of explaining both the cause of and the reason for the recurrence of certain common disorders of obscure aetiology in widely separated organs. It promised to show how totally different symptoms could be attributed to a common agent. The fact that the skin was sensitive but that the only clinical evidence of disease appeared, say, in the lungs gave the impression that all organs of the body must be in a like state of sensitivity. This has since been widely disputed and denied and on this subject Alexander (1936) states that such an assumption, at least from the clinical standpoint, is a fallacy. Man, he believes, may exhibit general hypersensitivity if a sufficiently large dose of protein be experimentally introduced.

If, then, hypersensitiveness is localized in certain tissues, it follows that the skin may or may not be sensitive in any given case and at any given time. In other words, because the ingestion of a specific food leads to colic or urticaria, it does not necessarily follow that the skin will give a positive reaction to an extract of that food in all cases or at all times. Indeed, a positive reaction may not occur at any time. This limitation of skin testing is now accepted; but nevertheless, allowing for the fact that only 50-60 per cent. of allergics (and that is not an over-generous estimate) give positive results, it still remains a very valuable diagnostic aid.