C4A and neoplasm: The MBL-associated serine proteases (MASPs) then cleave C4 and C2, forming the C3 convertase, which can initiate downstream complement activation and contribute to tumour cell lysis or immune modulation. sIgA is not a strong activator of complement, but it can interact with epithelial cells, tumour-associated antigens, and innate immune receptors, indirectly influencing tumour cell recognition.