I. Definition of acne Acne, also known as pimples. It is a chronic inflammatory disease of hair follicles and sebaceous glands. It occurs mainly on the face and back of the chest and other fatty areas, often accompanied by seborrhea. It is self-limiting and mostly heals or decreases after puberty. The factors that contribute to its pathogenesis include increased sebum production, hyperkeratosis of the epithelium at the mouth of the hair follicle and proliferation of Propionibacterium acnes (PA) in the hair follicle. Acne is closely related to excessive sebum production, obstruction of hair follicle sebaceous ducts, bacterial infection and inflammatory reactions. The pathophysiological basis of acne is the rapid development of sebaceous glands and excessive secretion of sebum, and the development of sebaceous glands is directly governed by androgens. After puberty, the level of androgens, especially testosterone, increases rapidly. Testosterone is converted to dihydrotestosterone in the skin by the action of 5-alpha reductase, which binds to androgen receptors in sebaceous gland cells. Increased androgen levels promote the development of sebaceous glands and the production of large amounts of sebum. Some patients with acne have higher blood levels of testosterone than those without acne. In addition, progesterone and dehydroepiandrosterone in the adrenal cortex also have a pro-sebum effect. Sebum is mainly composed of squalene, wax esters, triacylglycerols and small amounts of sterols and cholesterol esters. Acne patients have higher levels of wax esters and lower levels of linoleic acid in their sebum, and the reduced content of linoleic acid reduces essential fatty acids around the hair follicle and promotes keratinization of the follicle epithelium. Abnormal keratinization of the follicular sebaceous ducts is another important factor. Acne formation begins with the enlargement of the sebaceous follicles, and this enlargement is secondary to abnormal keratinization of the keratinocytes. In the lower part of the follicular funnel, the lamellar granules of keratin-forming cells are reduced and replaced by a large number of tension filaments, bridging granules, and lipid inclusion bodies. The secretion and discharge of large amounts of sebum are prone to bacterial infections. Various microorganisms such as Propionibacterium acnes, Staphylococcus albicans and Malassezia are present in the hair follicles, with Propionibacterium acnes infection being the most important. Propionibacterium acnes is an anaerobic bacterium, and the obstruction of sebum discharge creates a good local anaerobic environment for it to proliferate. The esterase produced by Propionibacterium acnes can break down triacylglycerols in sebum and produce free fatty acids, which are the main factors leading to inflammatory damage in acne. In addition, P. acnes can also produce peptides that chemotactic neutrophils, activate complement and cause leukocytes to release various enzymes, inducing or aggravating inflammation. In addition to the above factors, the occurrence of acne in some patients is also related to the immune function of the body, etc. Especially in some specific acne such as convergent acne and fulminant acne, the immune response plays an important role.