Objective To measure the quantities and apoptosis-related protein levels of B lymphocyte in the patients with immunorelated pancytopenia（IRP）and explore the action of B lymphocyte in the pathogenic mechanism of IRP. Methods Quantities of whole B lymphocytes and CD5+ B lymphocytes as well as the expressions of Fas and Bcl-2 in B lymphocytes in 35 patients with untreated IRP

15 IRP patients in complete remission （CR）

and 10 normal controls were assayed by flow cytometry. Results The percentages of B lymphocyte and CD5+ B lymphocyte were significantly higher in untreated IRP patients than in CR IRP patients and normal controls （P<0.05）

and there was no significant difference between the latter two groups （P>0.05）. There was no significant difference of Fas expression in B lymphocyte among three groups （P>0.05）. The expression of Bcl-2 in B lymphocyte was significantly higher in untreated patients than in CR patients or normal controls （P<0.01）

and significantly higher in CR patients than in normal controls （P<0.01）. The apoptosis-related index was significantly lower in untreated patients than in CR patients or normal controls （P<0.05）

and significantly lower in CR patients than in normal controls （P<0.05）. The percentage of B lymphocyte was positively correlated with post-treated response time（r=0.53

P<0.01）. Conclusion The production of auto-antibodies in IRP patients probably has some relationship with the abnormal quantities of B lymphocyte and its subpopulations as well as with the inhibition of B lymphocyte apoptosis.