Inflammatorylesions in acne include small red bumps (papules), pustules, large red bumps (nodules) and pseudocysts (these are fluctuant nodules). Inflammatory acne lesions are often painful.
An inflamed lesion usually follows rupture of the wall of a closed comedo. It may also arise from normal-appearing skin.
Inflammation follows immune activation in and around the pilosebaceous unit (hairfollicle and oil gland). Factors contributing to inflammation include:
Free fatty acids and sebum, which penetrate the dermis after the duct has ruptured and must be removed
Chemicals produced by acne bacteria such as Cutibacterium acnes, which diffuse into the surrounding skin (dermis)
Proinflammatorymediators (IL-1a, IL-b, TNF) produced by the cells lining the sebaceous ducts, ie the innate immune system
Debris within the follicles, which may induce a more severe granulomatous inflammatory reaction via macrophages and giant cells
Hypersensitivity or excessive immune reaction to P. acnes, the presumptive cause of the most severe form of acne, acne fulminans.
