The complement system is considered as a crucial mediator of the innate immune mechanisms. its three activation pathways contribute to cellular homeostasis, and protect the host from exogenous pathogens and self-derived components. The complement pathways are tightly regulated by surface-bound molecules and soluble proteins in order to avoid over-activation and pathological inflammatory response. However, tumor cells have evolved several strategies to escape from complement-dependent elimination and create a conducive microenvironment for tumor growth and metastasis. The excessive expression of complement proteins in malignant tumors and avoidance of membrane attack complex formation on its surface have been observed in almost all types of cancers. Complement cleavage products are found extensively deposited along the tumor vasculature, influencing the immunosuppressive tumor microenvironment. This chapter discusses the role of complement effectors and regulators in cancer immunity.
