The exact cause of plasmacytoid mastitis is still unclear. Most patients have no obvious cause for the onset of the disease, so it is thought to be an autoimmune disease, and the presumed causes are: 1. Poor drainage of the milk ducts due to obstruction of the large milk ducts. The main causes of milk duct obstruction are: nipple dysplasia, milk retention during lactation or difficulty in breastfeeding, inflammation, trauma and surgery in the areola area involving the milk ducts, and degeneration of the breast leading to degeneration of the myoepithelial cells of the milk ducts resulting in their contraction weakness. In addition, it may also be related to autoimmune and endocrine dysfunction. 2, peri-mammary duct inflammation: subareolar duct obstruction, causing duct dilation to a certain extent can cause epithelial atrophy of the duct wall, the duct wall is broken after the lipid-like and epithelial cell debris gathered in the duct corrodes the duct wall, epithelial debris and irritating phospholipids into the interstitium and lead to autoimmune reaction, a large number of plasma cell infiltration. 3, retrograde bacterial infection. The lactiferous duct is not open and can be secondary to bacterial infection, and the course of the disease is transformed from sterile inflammation to purulent inflammation. At the beginning of the disease can be only local red, then can form abscesses, chronic recurrent episodes, and finally feasible into sinus tracts. In summary, the obstruction and irritation of the milk ducts are the pathological basis for the development of the disease, while the bacterial growth that remains in the ducts at an early stage is an important factor in the secondary infection and aggravation of the development of the disease.