Sensitive skin terminology

Some of the terminology used when talking about sensitive skin, products and ingredients can be very confusing. Here is some Simple Skin-Terminology to help you make sense of sensitive skin.
Dermis - The dermis is a layer of skin below the epidermis and consists of connective tissue and cushions the body from stress and strain. 
Sensitivity – this is a result of a reaction to something. 
pH balanced –Human skin has a pH of 5.5.  Pure water has a pH very close to 7. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. pH balanced means that the formulation has been carefully produced to ensure it’s suitable for skin. 
Allergen - An allergen is any substance that can cause an allergy.
Irritant – something (e.g. an ingredient) which causes skin to become inflamed or painful. 
Rash - A rash is a change of the skin which affects its colour, appearance or texture. A rash may be localised in one part of the body or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell and may be painful. 
Hypoallergenic – a cosmetic or product which has been formulated specifically for skin which tends to be sensitive to reduce the chance of reaction. 
Dermatology - the branch of medicine dealing specifically with skin and its diseases. A doctor who specialises in this area is called a Dermatologist.  
Dermatologically tested – this means a product has been tested by a qualified dermatologist to ensure it is safe for use on skin. All Simple products are dermatologically tested. 
Skin allergy occurs when a chemical or other substance (called an allergen) gets through the barrier and is detected by the cells of the immune system. The first time the immune system detects an allergen nothing may happen, however, our immune system ‘remembers’ this allergen. The next time the same allergen gets through the barrier our immune cells recognise it immediately as something that should not be there and respond very quickly to remove it – this can result in a dramatic inflammatory reaction of the affected area of skin which could spread to other parts of the body. When your immune system has ‘remembered’ an allergen you have become ‘sensitised’ or ‘allergic’ to it. Once you are allergic to something you remain so for the rest of your life.
Skin sensitivity is different; the cells of the immune system do not get involved. When a chemical or substance (called an irritant) gets through the barrier the skins own cells detect and react to it. The reaction will usually occur the very first time the irritant gets through the barrier. An irritant reaction can look a bit like an allergic reaction because the skin may become red and sore, but as soon as you remove the irritant, the reaction will stop and your skin will quickly recover. You may come into contact with the ‘irritant’ many times but not get a reaction. Only when your barrier is weak and the irritant gets through will you react to it. In this case you are ‘sensitive’ to the irritant.
What makes a substance an allergen rather than an irritant is not fully understood, neither is why some people become allergic to a particular substance and others not.