CUTANEOUS REACTIONS TO ALLERGENS AND IRRITANTS 247 tion using them. Paraphenylene- diamine is a notable example. As a rule the stronger the concentra- tion of the sensitizer and the longer the exposure, the greater the chance of sensitivity and dermatitis. How- ever, continued exposure after sen- sitization may at times in some per- sons be followed by the development of hyposensitivity and tolerance. Besides the actual chemical causes of a contact dermatitis there are predisposing causes inherent in cer- tain individuals that play a part in their sensitivity to external irri- tants. Some of these factors are the type of skin, the degree of perspira- tion and the pH of the skin, and the amount of pigment. The defense mechanism of the skin can be altered by individual differences in these factors and make the skin more vulnerable to external irritants and sensitizers. This should be kept in mind by the cosmetic chemist in considering a formula for a product, and where on the skin and by whom it should be used. . Dermatitis caused by cosmetics is usually due to hypersensitivity, rarely to primary irritation, for any cosmetic may cause an allergic der- matiris in a hypersensitive person. However certain chemicals such as strong alkalis and resins, used in some cosmetics, are primary irri- tants, producing a dermatitis by their irritant action rather than because of the hypersensitivity of the individual. This type of re- action may occur with various prep- arations as depilatories, hair wavers and straighteners, hair lacquers, hair bleaches, solvents and anti- perspirants. Naturally these pri- mary irritants, even in mild concen- tration, can more readily sensitize the skin and also cause an allergic typeofdermatitis. There are many chemicals used in various •osmetic preparations which may produce a dermatitis by primary irritation and/or sensitization or by both. Schwartz and Peck (1) have com- piled a partial list of these chemicals and the cosmetics in which they are used. ALLERGY DEFINED As cosmetic dermatitis is usually due to hypersensitivity it might be well to discuss briefly the general theory of allergy and its mechanism in relation to contact dermatitis. Allergy is defined as any speci- fically acquired alteration in the capacity of living tissue to react ß to a substance. This altered capac- ity to react is dependent .upon an original or previous exposure to the same or an immunologically re- lated substance this casual sub- stance is termed an allergen. When the allergen can be demonstrated to produce antibodies it is called an antigem The phenomenon of al- lergy is usually explained as an antigen-antibody reaction. The al- lergen (antigen) supposedly pro- duces antibodies in the tissues and the interaction between them causes the clinical picture of allergy, like urticaria, eczema, hay fever, etc. Although at times antibodiei can be demonstrated in some types of cutaneous allergy by the method of

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS passive transfer, they have not conclusively been shown to be pres- ent in contact dermatitis. Passive transfer experiments by most inves- tigators have given negative results there is no proof that the reaction in an allergic contact type derma- titis, caused by chemicals, is based on the presence of specific antibodies and an antigen-antibody interac- tion. It is possible there are anti- bodies present in the affected skin but they have not been demon- strated with our present knowledge and laboratory techniques. How- ever certain substances may be found within the tissue which react with the allergen, or the allergen may react at the site of application to form a modified allergen which then reacts with specific antibodies in the skin to produce the dermatitis. As yet these theories have not been proved and the problem is still to be solved. At times there is a spread and even generalization of the dermatitis from its original site without con- tact of the specific sensitizing sub- s•ance to the adjacent or distant areas of the skin. This brings up the interesting problem of the route of the spread of this sensitization. Although absorption does occur through the intact skin and its appendages, it does not seem plaus- ible that the distant reaction is due alone to absorption of the antigen itself and its consequent hematog- enous dissemination. Several other routes have been propounded and experimentally demonstrated. In cases of dermatitis appearing near the site of actual contact, it has been shown that the sensitivity spreads centrifugally through the epidermis and cutis. The allergen or the anti- bodies, which cannot be demon- strated, in some way diffuse or cir- culate through the skin by means of so-called "trans-epidermal penetra- tion" (2). However, where the sensitivity becomes generalized there are probably several routes of absorption and dissemination. Ex- periments tend to show that the spread of the offending substance, whether it be the allergen itself or its modified form, or antibodies, occurs not only through the epider- mis and curls and blood vessels, but also and mainly through the lym- phatics (3). This concept seems to be the most logical explanation of the mechanism of the spread of the dermatitis. THE PATCH TEST An eruption due toga cosmetic usually begins abruptly and al- though it looks like a contact der- matiris, its appearance gives no particular clue to the specific cause. However by the history and several diagnostic criteria that are used by the experienced dermatologist, it is usually not difficult to decide whether a derrnatitis is due to a cos- metic. Along with these criteria the patch. test is a most important procedure, but to be of value it should be done correctly and inter- preted intelligently. The technique of doing a patch test is as follows: The cosmetic, in the form it is used, is placed on a