CD28 and autoimmune thrombocytopenic purpura: According to the analysis of the mean fluorescence intensity (MFI) of CD28, ITP patients with the CT genotype showed a higher level of CD28 protein expression than patients with the TT genotype (p = 0.006, Figures 2A, B), although the percentages of CD28+ cells in CD4+ T cells were all greater than 83% and showed no significant difference between the two groups (p > 0.05, Supplementary Figure 1).