衛生假說（英語：hygiene hypothesis）是一種醫學假說，指童年時因缺少接觸傳染源、共生微生物（如胃腸道菌群、益生菌）與寄生物，從而抑製了免疫係統的正常發展，進而增加了感染過敏性疾病的可能性。
This diagram simplifies the immune system into two separate tendencies: TH1 and TH2 responses. On the left, we see that exposures to germs, "dirt" and certain types of infection are part of the natural development of our immune response from a "default" TH2-based system at birth to a "mature" TH1-based system. On the right, we see how some cultural choices can interrupt the course of the immune system, and allow the TH2 response to continue to dominate and promote allergic conditions. Therefore, many of the advances of modernization, such as good sanitation and eradicating parasitic (helmith) infections, may actually be fueling this epidemic of allergies.
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a) 與皮膚屏障相關的基因突變使皮膚出現缺陷（水分流失，出現裂紋等），過敏原容易入侵，引起免疫反應。
b) 與免疫細胞受體/細胞因子相關的基因突變也影響免疫反應。
c) 環境因素
(3) 全景圖：瘙癢抓撓引起皮膚進一步損傷，導致感染， 病情加劇。控製瘙癢非常重要。
（4）進一步的研究說明表皮結構分子 filaggrin 等 以及IL-22 在濕疹中的作用。前者說明過敏皮膚保濕的重要。後者要轉換成療法還有很長的路要走。
