C1QA encodes the A chain of complement component C1q, a core structural component of the C1 complex that initiates the classical complement pathway. Together with C1QB and C1QC, C1QA recognizes and binds the Fc regions of IgG or IgM immunoglobulins complexed with antigens, activating the downstream proteolytic cascade that promotes pathogen phagocytosis and adaptive immune signaling. The C1Q subcomplex is activated by hexameric IgG complexes or pentameric IgM and also recognizes phosphatidylserine on apoptotic cells, linking complement activation to cell death clearance. Beyond classical immunity, C1QA participates in synaptic pruning by microglia during both development and disease. In progranulin-deficient neurodegeneration, C1qa deletion mitigates excessive synaptic elimination and behavioral pathology 1, and viral neuroinvasion upregulates C1QA in microglia to drive complement-mediated presynaptic terminal loss 2. In aging Alzheimer's disease models, C1qa expression marks disease-associated astrocytes and microglia; dopamine analogues reduce C1qa levels alongside improvements in neuroinflammation and cognitive function 3. C1QA expression associates with macrophage polarization states and disease contexts. Elevated C1QA characterizes pro-inflammatory tumor-associated macrophages in lung adenocarcinoma 4 and preeclampsia 5, and monocyte subsets expressing C1QA promote osteoclastogenesis in postmenopausal osteoporosis 6. Conversely, in skin cutaneous melanoma, C1QA overexpression correlates with better overall survival and enhanced immune infiltration 7, suggesting context-dependent prognostic significance.