We observed that MFAP5 + fibroblasts were the main senders that could release complement C3 to interact with the receptors ITGAM/ITGB2, ITGAX/ITGB2, and C3AR1 of C1QC + macrophages in tumor tissues, thus activating the complement system (Fig. 7A–C). This evidence concerns the gene C3 and neoplasm.