New preclinical findings in Nature Communications indicate that complement component C3 can shape immune checkpoint blockade efficacy by acting locally within the tumor microenvironment. The report links C3 production sites and downstream effects on myeloid infiltration, pointing to how immunotherapy response can depend on spatial biology rather than systemic complement levels. The work frames C3 as a conditional regulator—benefiting checkpoint responses only when the complement activity occurs within the tumor niche. That distinction matters for therapeutic design, biomarker strategy, and patient selection where complement modulation could otherwise risk broad immune effects. The results add mechanistic specificity to the growing overlap between complement biology and tumor immunology, particularly around myeloid cell recruitment and function.