Complement C1q subcomponent subunit A

C1QA encodes complement C1q subcomponent subunit A, a core component of the C1 complex that initiates the classical complement pathway. Together with C1QB and C1QC, it recognizes and binds the Fc regions of immunoglobulin G or M complexed with antigens, triggering a proteolytic cascade that leads to pathogen phagocytosis and adaptive immune signaling (UniProt: P02745). The protein also recognizes phosphatidylserine on apoptotic cell surfaces, potentially promoting complement activation during programmed cell death.

C1QA is broadly relevant to immune and inflammatory processes. Mutations causing C1q deficiency result in impaired classical pathway activation and increased susceptibility to immune complex disease, chronic infection, and systemic lupus erythematosus (UniProt: P02745, MIM 613652).

C1QA is upregulated in Alzheimer's disease brain tissue. Analysis of post-mortem AD brain versus age-matched controls using quantitative proteomics detected a mean log2 fold-change of +0.53 (Chaparral AD proteomics). This elevation aligns with evidence of neuroinflammatory complement activation in AD pathology and suggests C1QA may contribute to disease-associated immune responses in the AD brain.