C1QB

C1QB is a protein that is part of the complement system, a crucial component of the innate immune system. Specifically, C1QB is a subunit of the C1 complex, which is the first recognition and activation molecule of the classical pathway of complement. The C1 complex is a large, calcium-dependent macromolecular machine composed of three distinct protein entities: C1q, C1r, and C1s. C1QB, along with C1QA and C1QC, forms the globular head regions of the C1q molecule.

The primary role of C1q, and therefore C1QB, is to initiate the complement cascade by binding to antibody-antigen complexes or directly to certain microbial surfaces. This binding event triggers a conformational change in the C1 complex, leading to the activation of the serine proteases C1r and C1s. These activated proteases then cleave downstream complement components, initiating a cascade of enzymatic reactions that results in the elimination of pathogens through various mechanisms, including opsonization, inflammation, and direct cell lysis. Mutations in C1QB can lead to deficiencies in C1q, which are associated with an increased susceptibility to certain autoimmune diseases, particularly systemic lupus erythematosus.