Here, we used an integrative multi-omics analysis of clinical data to elucidate the association between the complement component (C3, C5, C3AR1, and C5AR1) and dysfunctional T-cell phenotypes, tumor immune evasion, therapy outcome, and prognosis in various cancer types, placing much emphasis on the effect of genetic and epigenetic alterations in the cancer-associated biological activities of the complement. The gene discussed is C3AR1; the disease is cancer.