LAUSR

higher prevalence of smokers among patients with AA. Meta-analysis showed an association between ever smoking and AA, with a pooled OR of 1.12 (95% CI 1.10–1.14), Figure 1. Heterogeneity across studies was I = 48%. There was no evidence of publication bias according to the funnel plot and Egger’s regression test (P = 0.37). Association was also found in a subgroup analysis that investigated current smoking (OR 1.14, 95% CI 1.03–1.26); however, data were available in only two studies (Lee 2019, Dai 2020). The association between smoking and AA has recently been investigated by Dai et al. They found that current smokers had a higher risk of incident AA than never smokers (adjusted hazard ratio 1.88, 95% CI 1.22–2.88) and a trend toward increased risk with increasing numbers of years of smoking and cumulative pack-years. Despite these findings, the exact mechanism by which cigarette smoking influences hair follicles remains uncertain, and various theories have been proposed. Smoking has a substantial influence on both humoral and cell-mediated immune responses. As a result, several medical conditions have been found to be associated with chronic exposure to cigarette smoke. Smoking was shown to induce the release of various cytokines, including TNF-a, interleukin (IL)-1, and IL-6. Those cytokines may play a role in the pathogenesis of AA by eliciting an inflammatory response around the hair follicle. Hair follicles are one of the tissues that exhibit “immune privilege,” which protects them from systemic immune attack. The collapse of hair follicle immune privilege may result in AA. Furthermore, cigarette smoke contains a high concentration of free radicals, which may build up in the hair follicle, eventually leading to a breakdown of the immune privilege. The present study is not without limitations. Only a small number of studies were included in the analysis. Moreover, the chronology of the onset of smoking and AA was not available, making conclusions difficult to reach. An additional limitation is the lack of data regarding the influence of cessation of smoking on AA. In conclusion, an association was found between smoking and AA. Further well-designed studies are warranted to elucidate the effect of smoking and to find additional risk factors for AA.