Among 56 patients with VTE, 14 had decrease of CD16 + +56 + NK cells, which suggests the ability of NK cells to kill intracellular pathogens including virus is impaired. CD19 is only expressed on the B lymphocytes of normal hemopoietic system and the follicular dendritic cells (FDC) of germinal center. The expression of CD19 is detectable back to progenitor B cells and present during the maturation of B lymphocytes. Once the B cells differentiate into plasma cells, the CD19 expression is absent. CD19 involves in the flux of Ca 2+ in the B lymphocytes, and can regulate the activation and proliferation of B cells (17) . Among 56 VTE patients, 15 had decreased CD19 expression, which suggests the activity and proliferation of B cells are compromised.