Fig. 1

Changes in bone marrow/thymus accompanying aging and changes in immune response cells. Although the number of stem cells in the bone marrow is not affected by aging, differentiation into common lymphoid progenitor cells decreases and shifts to differentiation into myeloid-type common progenitor cells. Therefore, differentiation into lymphoid cells (T cells, B cells) decreases, and differentiation into myeloid cells (granulocytes/monocytes) increases. The thymus, which is the site of the differentiation and maturation of T cells, atrophies with age. Therefore, in young people, naive T cells predominate; however, with age, there is a shift to dominant T cells (memory T cells), which is activated by antigen stimulation or some internal factor. HSCs, hematopoietic stem cells; CMP, common myeloid progenitor; CLP, common lymphoid progenitor