C1QA encodes the A chain of complement component C1q, a core structural component of the C1 complex that initiates the classical complement pathway 1. C1QA, together with C1QB and C1QC, recognizes and binds the Fc regions of IgG or IgM antibodies complexed with antigens, activating the complement cascade leading to pathogen phagocytosis and immune signaling 234. Immunoglobulin binding activates the C1R proenzyme, which cleaves C1S to initiate the downstream proteolytic cascade 4. C1Q requires a hexamer of IgG or pentameric IgM for full activation 34. Beyond classical pathogen immunity, C1QA participates in homeostatic functions including recognition of phosphatidylserine on apoptotic cells and complement-mediated synaptic pruning by microglia 56. C1QA is upregulated in alternatively activated macrophages and expanded monocyte populations during infection and inflammatory conditions 78. C1QA dysfunction is implicated in multiple diseases: neurodegeneration through aberrant synaptic pruning 6, atherosclerosis and stroke risk 9, and connective tissue disorders showing reduced serum C1QA levels 10. Dysregulation of C1QA expression is also associated with Alzheimer's disease pathology 11 and enhanced immune responses in fibrotic conditions like keloids 12.