Objective To investigate the expression and clinical significance of T lymphocyte subsets, natural killer (NK) cells and CD19+ B cells in the elderly with primary immune thrombocytopenia (ITP) before and after treatment. Methods The elderly ITP patients diagnosed and treated in the Songjiang Hospital Affiliated to Shanghai Jiaotong University School of Medicine (preparatory stage) between January 2014 and June 2019 were retrospectively selected as the observation group. The healthy elderly in the same period were selected as the control group. According to the treatment, the observation group was divided into effective group and ineffective group. The expression levels of T lymphocyte subsets (CD3+, CD4+, CD8+ and CD4+/CD8+), NK cells and CD19+ B cells were observed and analyzed. Results A total of 75 subjects were included, including 35 in the observation group and 40 in the control group. The total effective rate was 85.71% (30/35). Before treatment, the expression levels of T lymphocyte subsets (CD3+, CD4+ and CD4+/CD8+) in the observation group were lower than those in the control group (P<0.05). There was no significant difference in other indexes between the two groups (P>0.05). After treatment, except for CD8+, the expression levels of T lymphocyte subsets (CD3+, CD4+ and CD4+/CD8+) in the observation group were higher than those before treatment (P<0.05). The expression levels of NK cells and CD19+ B cells were lower than those before treatment (P<0.05). The expression levels of T lymphocyte subsets (CD3+, CD4+ and CD4+/CD8+) in the effective group were higher than those before treatment (P<0.05), while the expression level of CD19+ B cells was lower than that before treatment (P<0.05). There was no significant difference in other indexes before and after treatment (P>0.05). There was no significant difference in the expression levels of T lymphocyte subsets (CD3+, CD4+, CD8+ and CD4+/CD8+), NK cells and CD19+ B cells in the ineffective group before and after treatment (P>0.05). Conclusions T lymphocyte subsets are abnormal in elderly ITP patients. The immune abnormality of T lymphocyte may be one of the reasons for elderly patients with ITP. With the improvement of therapeutic effect, immune cell subsets have also been improved.