The complex C1 triggers the activation of the Complement classical pathway through the recognition and binding of antigen–antibody complex by its subunit C1q. The globular region of C1q is responsible for C1 binding to the immune complex. C1q can also bind nonimmune molecules such as DNAand sulfated polysaccharides, leading either to the activation or inhibition of Complement. The binding site of these nonimmune ligands is debated in the literature, and it has beenproposed to be located either in the globular region or in the collagen-like regionof C1q, or inboth
