including the umbilical cord, endometrial polyps, menstrual blood, bone marrow, adipose tissue, etc. [9]. MSCs are currently the most common cell type in regenerative medicine. Numerous studies have demonstrated the potential benefits of MSC-based therapies for the treatment of various pathologies, such as bone and cartilage diseases, cardiac ischemia, diabetes, and neurological disorders [11]. In addition to numerous advantages and benefits, there are a number of problems limiting the widespread use of MSC therapy in clinical practice, which include the risk of developing tumors and transmitting viruses and prions after stem cell transplantation; how long-term in vitro cultivation of MSCs leads to loss of the potential of MSCs for differentiation and morphological changes and also increases probability of malignant transformation; low survival and engraftment rates of MSCs due to their short-lived viability after injection; the low therapeutic effect and increased immunogenicity in differentiated MSCs; the heterogeneity of MSCs due to differences in the health status, genetics, gender, and age of donors; varying degrees of stem cell stability and differentiation capacity between MSCs isolated from different sources; different levels of expansion ability under different culture conditions; immune compatibility between donors and recipients; ethical issues; and high manufacturing costs [12; 13; 14; 15]. Consequently, long-term research and monitoring are required to examine the long-term effects of MSC therapy, including any negative effects [12].  
Along with MSCs, the therapeutic effects of their exosomes (MSC-Exo) are being extensively investigated. This is mostly because stem-cell-derived exosomes have various advantages over stem cells, including non-immunogenicity, non-infusion toxicity, easy access, easy storage, and the absence of tumorigenic potential and ethical problems [16]. Exosomes are lipid bilayer vesicles that are spherical and have a diameter that varies from 30 to 150 nm. Exosomes exhibit selective enrichment for distinct biomolecules, such as membrane glycoproteins, lipids, and other cell-specific proteins, in addition to multiple types of nucleic acids [17]. Exosomes are involved in intercellular communication through a variety of cargo types and can influence the immune response by interacting with immune effector cells in the presence of anti-inflammatory compounds [18]. MSC-Exo are used to mediate tissue regeneration in a variety of diseases, such as cutaneous wound healing [19], cancer [20; 21; 22], ischemic heart disease, lung diseases [23], liver fibrosis, and neurological, autoimmune, and inflammatory diseases [18]. As already mentioned, a significant advantage of MSC-Exo compared to MSCs is that they overcome the limitations of cell therapy by providing comparable benefits in a safer and more stable extracellular vesicle format. Moreover, the contents of exosomes can also be modified to enhance regenerative biological activity [24].  
In this regard, it is not surprising that the scientific community is interested in studying the therapeutic properties of MSCs and MSC-Exo in the treatment of psoriasis. This review summarizes the recent advancements from preclinical and clinical studies of MSCs and MSC-Exo in the treatment of psoriasis, and it also discusses their mechanisms of therapeutic action involved in the treatment of this disease.