Mesenchymal stem cells and their derived exosomes for the treatment of COVID-19

Table 3 Coronavirus disease 2019 causes secondary pathological changes

Organ system	Secondary change
Spleen	The spleen shrinks. The white marrow is atrophic, with a decreased number of lymphocytes and some cell necrosis; the red marrow is congested and focally hemorrhagic, macrophages are proliferated, and phagocytosis is observed in the spleen; anemic infarcts of the spleen are easily found. Immunohistochemical staining shows decreased spleen CD4+ T and CD8+ T cells[144]	+
Lymph nodes	The lymphocyte count is reduced, and necrosis is found. Immunohistochemical staining shows decreased CD4+ T and CD8+ T cells in the spleen and lymph nodes. Lymph node tissues may be positive for novel coronavirus nucleic acid detection in macrophages[145]	+
Bone marrow	Hematopoietic cells are either hyperplastic or reduced in number, with an increased granulocyte-red ratio[146]

+: The nucleic acid test for novel coronavirus was positive.