PDCD1 and neoplasm: Some scholars found that C3d enhances antitumor immunity independently of B cells, natural killer cells (NK cells), or antibodies, but it does so by increasing tumor infiltrating CD8+ lymphocytes, by depleting Tregs, and by suppressing expression of programmed cell death protein 1 (PD-1) by T cells, which hint the potential of C3d to be used as a marker for tumor staging and patient prognosis in lymphoma (Elvington et al., 2014).