Rapid clearance of adoptively transferred Cd47-null (Cd47^−/−) cells in congeneic WT mice suggests a critical self-recognition mechanism, in which CD47 is the ubiquitous marker of self, and its interaction with macrophage signal regulatory protein α (SIRPα) triggers inhibitory signaling through SIRPα cytoplasmic immunoreceptor tyrosine-based inhibition motifs and tyrosine phosphatase SHP-1/2. However, instead of displaying self-destruction phenotypes, Cd47^−/− mice manifest no, or only mild, macrophage phagocytosis toward self-cells except under the nonobese diabetic background. Studying our recently established Sirpα-KO (Sirpα^−/−) mice, as well as Cd47^−/− mice, we reveal additional activation and inhibitory mechanisms besides the CD47-SIRPα axis dominantly controlling macrophage behavior. Sirpα^−/− mice and Cd47^−/− mice, although being normally healthy, develop severe anemia and splenomegaly under chronic colitis, peritonitis, cytokine treatments, and CFA-/LPS-induced inflammation, owing to splenic macrophages phagocytizing self-red blood cells. Ex vivo phagocytosis assays confirmed general inactivity of macrophages from Sirpα^−/− or Cd47^−/− mice toward healthy self-cells, whereas they aggressively attack toward bacteria, zymosan, apoptotic, and immune complex-bound cells; however, treating these macrophages with IL-17, LPS, IL-6, IL-1β, and TNFα, but not IFNγ, dramatically initiates potent phagocytosis toward self-cells, for which only the Cd47-Sirpα interaction restrains. Even for macrophages from WT mice, phagocytosis toward Cd47^−/− cells does not occur without phagocytic activation. Mechanistic studies suggest a PKC-Syk–mediated signaling pathway, to which IL-10 conversely inhibits, is required for activating macrophage self-targeting, followed by phagocytosis independent of calreticulin. Moreover, we identified spleen red pulp to be one specific tissue that provides stimuli constantly activating macrophage phagocytosis albeit lacking in Cd47^−/− or Sirpα^−/− mice.