C1QC is the C chain component of complement C1q, a core subunit of the C1 complex that initiates the classical complement pathway 1. Together with C1QA and C1QB, C1QC recognizes and binds the Fc regions of IgG or IgM antibodies complexed with antigens on pathogen surfaces, with activation occurring upon hexameric IgG or pentameric IgM binding 23. This immunoglobulin binding activates the proenzyme C1R, which cleaves C1S and initiates the proteolytic complement cascade leading to pathogen phagocytosis and adaptive immune signaling 3. C1QC also binds phosphatidylserine on apoptotic cells, potentially promoting complement activation during programmed cell death 4. Beyond classical complement activation, C1QC serves as a marker for distinct immune cell populations in pathological contexts. C1QC-expressing macrophages have been identified in atherosclerotic plaques, where genetic analysis suggests a positive association between C1Q and ischemic stroke risk 5. In cancer microenvironments, lipid-associated macrophages and tumor-associated macrophages characterized by high C1QC expression facilitate disease progression through enhanced phagocytosis and immunosuppression 67. C1QC is also upregulated in fibrotic conditions like keloids and appears in plasma exosomes as a potential biomarker for Alzheimer's disease 89. Deficiency in C1q components causes C1q deficiency disease. Overall, C1QC functions both in canonical immune activation and as a marker of specialized myeloid cell populations in disease.