Complement C1q subcomponent subunit B

C1QB (Complement C1q subcomponent subunit B) is a core component of the complement C1 complex, which initiates the classical pathway of the complement system. Together with C1QA and C1QC, C1QB recognizes and binds the Fc regions of IgG or IgM immunoglobulins complexed with antigens, triggering a proteolytic cascade that leads to pathogen phagocytosis and adaptive immune signaling (UniProt: P02746). The protein also binds phosphatidylserine on apoptotic cells, potentially promoting complement activation during programmed cell death.

C1QB deficiency causes C1q deficiency 2 (C1QD2), an autosomal recessive immunodeficiency characterized by severe immune complex disease, recurrent infections, and increased systemic lupus erythematosus risk (UniProt: P02746). The protein's role in complement-mediated immune clearance and its association with autoimmune disease underscore its importance in immune homeostasis.

In Alzheimer's Disease, C1QB is significantly upregulated in post-mortem AD brain tissue compared to age-matched controls (mean log2FC = 1.19; Chaparral AD proteomics). This elevation is consistent with complement system activation in AD pathology, which is hypothesized to contribute to neuroinflammation and neurodegeneration through excessive clearance of synaptic components and activation-induced neuronal damage.