In 2011, we reported the proportions of CD3+T cells and CD8+T cells were markedly reduced and CD4/CD8 significantly increased in a series of CTEPH patients, which suggests the compromised T cell immunity in CTEPH patients and imbalance between CD3+T cells and CD8+T cells (26) . In addition, we also found that the number of CD3+T cells and CD8+T cells was dramatically reduced in a series of acute PE patients and cytological findings also supported the results from genomics studies on PE patients (27) . The occurrence of PE is related to the deficient or compromised T cell mediated immunity. This deficiency of T cell immunity may occur under the following conditions: 1) viral infection; 2) malignant tumors; 3) medication with immunosuppresants; 4) malnutrition. In the present study, most of subjects were old patients and malnutrition was not clinically obvious. Moreover, malignant tumors were not found in these patients and medication with immunosuppressants was absent. Thus, the compromised T cell immunity might be possibly related to viral infection.