Several immune biomarkers have demonstrated prognostic significance: soluble CD40 ligand (sCD40L) predicts thromboembolic complications in chronic AF (Duygu et al. [20]); CD4+CD28− T cells independently predict cardiovascular mortality in patients with AF and heart failure (Sulzgruber et al. [21]); and aberrant CD40/CD40L axis signaling is associated with increased thrombotic risk (Ogurkova et al. [19]). Here, CD4 is linked to heart failure.