The circulating CD4/CD8 ratio is an adverse prognostic factor in refractory/relapsed DLBCL–PCNSL (primary central nervous system lymphoma) (Ghesquieres et al., 2019), mantle cell lymphoma (Nygren et al., 2014), and Waldenstrom macroglobulinemia (Cao et al., 2021) in the HIV-negative population. Here, CD4 is linked to primary central nervous system lymphoma.