The presence of clogged skin follicles (known as comedones) limited to the face with occasional inflammatory lesions defines mild acne. Moderate severity acne is said to occur when a higher number of inflammatory papules and pustules occur on the face, compared to mild cases of acne, and appear on the trunk of the body. Severe acne is said to occur when nodules (the painful 'bumps' lying under the skin) are the characteristic facial lesions, and involvement of the trunk is extensive. Large nodules were previously called cysts. The term nodulocystic has been used in the medical literature to describe severe cases of inflammatory acne. True cysts are rare in those with acne, and the term severe nodular acne is now the preferred terminology. Acne inversa (L. invertō, "upside-down") and acne rosacea (rosa, "rose-colored" + -āceus, "forming") are not forms of acne and are alternate names that respectively refer to the skin conditions hidradenitis suppurativa (HS) and rosacea. Although HS shares certain overlapping features with acne vulgaris, such as a tendency to clog skin follicles with skin cell debris, the condition otherwise lacks the hallmark features of acne and is therefore considered a distinct skin disorder. Signs and symptoms Typical features of acne include increased secretion of oily sebum by the skin, microcomedones, comedones, papules, nodules (large papules), pustules, and often results in scarring. The appearance of acne varies with skin color. It may result in psychological and social problems. Scars Acne scars are caused by inflammation within the dermis and are estimated to affect 95% of people with acne vulgaris. Abnormal healing and dermal inflammation create the scar. Scarring is most likely to take place with severe acne but may occur with any form of acne vulgaris. Acne scars are classified based on whether the abnormal healing response following dermal inflammation leads to excess collagen deposition or loss at the site of the acne lesion. Atrophic acne scars have lost collagen from the healing response and are the most common type of acne scar (accounting for approximately 75% of all acne scars). Ice-pick scars, boxcar scars, and rolling scars are subtypes of atrophic acne scars. Boxcar scars are round or ovoid indented scars with sharp borders and vary in size from 1.5–4 mm across. Ice-pick scars are narrow (less than 2 mm across), deep scars that extend into the dermis. Rolling scars are broader than ice-pick and boxcar scars (4–5