The few studies investigating the role of complement proteins in mammary tumor models have yielded results that are in contrast to the majority of studies in other murine cancer models which show that pharmacological blockade of C5aR1 or C3aR inhibits tumor progression by limiting recruitment of immunosuppressive myeloid cells and Tregs into the tumor and promoting effective T cell responses [24,25,27,28,30]. Here, C5AR1 is linked to breast cancer.