CD4 and neoplasm: An analysis of the frequency of occurrence of PD-1-positive on selected lymphocyte subpopulations in the tested biological material in relation to the serum iron level showed a significant negative correlation between T CD8 + PD-1+ lymphocytes (p = 0.023) in tumor samples and T CD4+ PD-1+ lymphocytes (p = 0.000) in node samples.