Complement component 1q (C1q)

Complement component 1q is a large pattern recognition protein that forms part of the **C1 complex**, initiating the **classical pathway** of the complement system—an essential arm of innate immunity. Structurally, it consists of 18 polypeptide chains arranged into six heterotrimers with collagen-like regions forming a bouquet-like structure topped by globular heads responsible for ligand binding[2][4]. Upon recognizing antigen-antibody complexes—primarily IgM and most IgG subclasses—C1q triggers conformational changes leading to activation cascades involving serine proteases **C1r** and **C1s**, ultimately resulting in opsonization, cell lysis, phagocytosis promotion, and modulation/removal of immune complexes from circulation[6]. Beyond its canonical role in immunity, **C1q** has diverse non-canonical functions including synaptic pruning during neurodevelopment,[3] regulation/modulation within stem cell biology,[3] proangiogenic effects supporting tissue repair/tumor growth,[7] direct interactions with various cell-surface receptors,[3] clearance/prevention against autoantigen exposure through apoptotic cell removal,[4], [5], [6], [9]. Dysregulation or deficiency is implicated in autoimmune disorders like lupus erythematosus as well as neurodegenerative conditions.[3] No approved therapies specifically inhibit/target only C1q; most clinical interventions focus on blocking downstream effectors within its cascade.

Other names

Complement protein C1qC1qComplement component 1q subcomponentFirst component of complement subcomponent q

For drugs targeting this molecule or its pathway: - Inhibition/blockade of classical complement activation by preventing interaction with antibody-antigen complexes or downstream proteases. Most current approaches target associated proteins such as C1s rather than directly inhibiting/binding to C1q itself.