CD4 and B-cell chronic lymphocytic leukemia: A similar trend was found for PD-1: its expression was significantly higher on CD4+, CD8+, and CD19+ lymphocytes in both patient groups relative to HV (p < 0.001), with CLL with SID demonstrating the highest PD-1 levels, particularly in the CD4+ subset (p = 0.003 vs. CVID; Figure 1G–I).