The MR analysis revealed that CD28 on CD28+CD45RA+CD8br was significantly associated with genetic susceptibility to T1D, while EM DN (CD4−CD8−) %T cells, EM CD8br %T cells, IgD+CD38dim %lymphocytes, CD80 on monocytes, SSC-A on pDCs, and FSC-A on pDCs were potentially associated with genetic susceptibility to T1D. This evidence concerns the gene CD28 and type 1 diabetes mellitus.