Our analysis of the differential expression between tumor and adjacent normal tissue, and the genetic as well as epigenetic modulation, of the complement components C3, C5, C3AR1, and C5AR1 not only demonstrates that the complement components are biomarkers of cancer progression and prognosis but also suggests that these complement components are associated with tumor immune infiltration and immune invasion and could be used to predict patients that would benefit from certain therapies in multiple cancer types. This evidence concerns the gene C3 and cancer.