Cells of CTLA-4+ and PD-1+ in SCLC peripheral blood were (1.56±1.24)% and (8.07±3.97)%; there is no significant difference between CD3+CTLA-4+ and CD4+CTLA-4+, (4.87±5.18)% and (3.85±2.60)%, but show lower expression than CD3+PD-1+ and CD4+PD-1+ (26.63±9.04)% and (20.79±9.41)%, respectively. Here, PDCD1 is linked to small cell lung carcinoma.