Acne lesions frequently contain pus. This close-up photograph shows small acne pustules with surrounding inflammation (erythema).

Blackheads, or open comedones, are common in acne. Clogged hair follicles reflect light irregularly to produce this black hue.

Blackheads, or open comedones, are common in acne. Clogged hair follicles reflect light irregularly to produce this black hue.

Cystic acne may occur across the upper chest as well as on the back.

The face is the most common location of acne. Here, there are 4 to 6 millimeter red (erythematous) pustules, some with bridging scars and fistulous tract formation (connecting passages). Severe acne may have a profound psychological impact and may cause scarring. Effective treatments are available for this type of acne.

Acne frequently occurs on the back. Here, there are 2 to 6 millimeter wide erythematous (red) pustules with large open and closed comedones. Permanent scarring may follow a severe case of acne. Men are more often affected on their shoulders and back than are women.

Cystic acne, or nodulocystic acne, is the most severe form of acne. Both pustules and hard red bumps are present in the skin. This form of acne is more difficult to treat and often requires taking an oral vitamin A derivative.

Cystic acne, or nodulocystic acne, is the most severe form of acne. Both pustules and red bumps are present in the skin and scarring can be seen. The back is a common site for acne.