CD4 and lung cancer: Since CD8+ cytotoxic T lymphocytes (CTLs), CD4+ T helper cells and CD3−/CD56+ natural killer (NK) cells are key players in anti-tumor immunity, and the immunosuppressive CD4+/CD25+/FoxP3+ regulatory T cells (Tregs) can contribute to NK and T cell exhaustion [27,28], the question arises as to whether alterations in the composition of these immune cell types in the peripheral blood has any prognostic relevance for the prediction of therapeutic success and the overall survival of patients with lung cancer [28,29].