1.1.2.5　Cells

The resident cells in the dermis mainly include fibroblasts，macrophages，and mast cells.They are mainly distributed in the papillary layer of the dermis，around the blood vessels under the papillary layer，and between the collagen fiber bundles.The dermis also contains a small amount of dermal dendritic cells and lymphocytes.Fibroblasts are derived from the mesoderm and can synthesize and degrade fiber and matrix components.They play an important role in dermal network construction and epidermal connection.Macrophages are derived from bone marrow hematopoietic stem cells and differentiate into circulating monocytes and migrate to the dermis to differentiate into macrophages，which have the effects of phagocytosis，antigen presentation，killing pathogenic microorganisms and tumor cells.Mast cells can synthesize and release inflammatory mediators，such as histamine，heparin，trypsin，and participate in type Ⅰ hypersensitivity.Dermal dendritic cells are specialized antigen-presenting cells in the dermis.They can efficiently recognize and present antigens，synthesize and secrete cytokines，and migrate to local draining lymph nodes to activate T cells to induce immune responses.(https://www.daowen.com)