Acne affects mainly the face, although other regions rich in sebaceous glands can also be affected (chest, back, upper arms). The lesions can be distinguished into non-inflammatory (open and closed comedones or blackheads and whiteheads) and inflammatory lesions (papules, pustules and nodules).
Four main factors are known to influence the development of acne, namely: (i) sebaceous gland hyperplasia with excess sebum production (seborrhea); (ii) follicular epidermal hyperproliferation and altered differentiation; (iii) follicular colonization by Propionibacterium acnes (P. acnes); and (iv) inflammation and immune response. Altered epidermal growth and differentiation, combined with seborrhea, is responsible for the formation of the primary lesion in acne: The microcomedo. The development of inflammatory lesions, instead, is often triggered by the effects of P. acnes with release of inflammatory mediators.
The first entity in the development of an acne lesion is a tiny invisible plug (microcomedone) of the pilosebaceous duct; skin that is at one time apparently unaffected may subsequently develop lesions if not treated. Generally speaking, it may take up to four weeks for an untreated papule or pustule to complete its life cycle from start to end. Therefore, an acne therapy that significantly reduces lesion counts during the first four weeks is recognized as having treated existing lesions, while therapies effective in reducing acne lesions count during the following four weeks are considered also effective in preventing the appearance of new lesions.
Because of the multiple pathogenetic factors, dermatologists recommend treating acne with combinations of agents that act at different levels. It is widely recognized that an effective acne treatment should not only clear current acne lesions but also prevent the appearance of new ones.