On every tube, bottle or jar of a cosmetic product you will find the ingredients of the product listed. This is regulated by law in order to give professionals like cosmetic chemists and dermatologists an idea about the mode of action of the active ingredients. People sometimes show allergic reactions after using certain cosmetic products even when these products claim to be hypo-allergenic. In most cases a dermatologist will be able to detect the ingredient that the person is allergic to and will then recommend cosmetic products without this ingredient. This is the most important reason for the ingredients to be listed on the product container.
Allergic reactions are very complex but today they are relatively well understood.
A prerequisite of an allergic reaction is that the allergenic substance is able to penetrate into the deeper layers of your skin, the so-called epidermis. This substance may there be able to sensitize special types of cells that then show adverse reactions by producing substances to overcome these reactions which are the releasing factors for the allergic reaction. This complex chemical mechanism normally does not happen the first time when you come in contact with the cosmetic product. In most cases the allergic reaction is not caused by the cosmetic product itself but by products, similar to the current cosmetic product, that you were in contact with in the past. That product sensitizes you for the cosmetic product that in turn gives you the allergic reaction.