CD4 and B-cell chronic lymphocytic leukemia: In addition to the decreased frequency of CD3+/CD4+/CD28+ and CD3+/CD8+/CD28+ cells in B-CLL patients, the MFI of the CD28+ cells, as a measure of the antigen density on the cell surface, was also lower in patients at 48 and 72 h on CD4+ T cells and 72 h on CD8+ T cells after stimulation than in the controls.