The presence of elevated levels of complement C1q, C3b, C4d, and Bb, and the release of anaphylatoxins that engage C3aR1 and C5aR1 expressed by microglia, may stimulate functional pathways leading to the induced expression of inflammatory cytokines and phagocytic processes in the MS grey matter. Here, C3AR1 is linked to myeloid sarcoma.