The classical pathway plays a role in both innate and adaptive immunity.C1q family proteins are characterized by a distinctive 'globular domain' of about 140 amino acids (the gC1q domain) situated at the carboxyl terminus of a collagen 'stalk'. They often form a characteristic superstructure in which three protomers trimerize to form a collagen triple helix, and these trimers multimerize to form a ‘bouquet’. Complement component C1q, the first component of this pathway, links the adaptive humoral immune response to the complement system by binding to antibodies complexed with antigens. C1q can, however, also bind directly to the surface of certain pathogens and thus trigger complement activation in the absence of antibody. C1q is part of the C1 complex, which comprises a single C1q molecule bound to two molecules each of the zymogens C1r and C1s. Complement component C1q is a calcium-dependent sugar-binding protein, a lectin, belonging to the collectin family of proteins, which contains both collagen-like and lectin domains hence the name collectin. It has six globular heads, linked together by a collagen-like tail, which surround the (C1r:C1s)2 complex. Binding of more than one of the C1q heads to a pathogen surface causes a conformational change in the (C1r:C1s)2 complex, which leads to activation of an autocatalytic enzymatic activity in C1r; the active form of C1r then cleaves its associated C1s to generate an active serine protease.
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