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C7
C7 Full Name
complement component 7
C7 Introduction
Complement component 7 (C7) is a key serum glycoprotein in the terminal pathway of the complement system, essential for the formation of the membrane attack complex (MAC) and the execution of cytolytic function. Structurally, C7 is a single-chain glycoprotein composed of approximately 821 amino acid residues and contains an N-terminal signal peptide. The protein architecture of C7 is complex, encompassing multiple functional domains such as the MACPF (membrane attack complex/perforin) domain and TSP (thrombospondin-type) domain, which are critical for its subsequent interactions with other complement components.
The final steps of complement activation begin when C5 convertase cleaves C5 to generate the C5b fragment. Newly formed C5b first binds to C6, forming a relatively stable, water-soluble C5b6 complex. The subsequent binding of C7 marks a turning point in the process. When a C7 molecule associates with the C5b6 complex, it induces a dramatic conformational change in the resulting ternary complex (C5b-7). This change exposes previously hidden hydrophobic regions within the complex, transforming it from hydrophilic to amphiphilic and enabling it to interact with lipid bilayers. It is precisely this critical action of C7 that allows the C5b-7 complex to dissociate from its site of formation and, as the first membrane-anchoring unit of the MAC, irreversibly insert into the membrane of a nearby target cell. Once C5b-7 is stably embedded in the membrane, it becomes a high-affinity receptor for recruiting and binding the next component—C8. The binding of C8 further anchors the complex deeper into the membrane and ultimately catalyzes the polymerization of multiple (typically 10–16) C9 molecules to form a transmembrane tubular pore, the complete MAC. Immunologically, C7 is an indispensable component in complement-mediated cytolysis. Its presence ensures that the terminal defense mechanism of the complement system can be fully executed, enabling direct perforation of bacterial or compromised host cell membranes via MAC formation for rapid immune clearance.
Figure 1. A simplified representation of the complement system. (Source: Okrój M, et al. 2019)
Due to C7's key position in the complement cascade, functional defects in C7 impair the entire MAC assembly process, thereby weakening humoral immune defense. Hereditary C7 deficiency is a rare autosomal codominant primary immunodeficiency, whose core pathophysiological consequence is the inability to form a functional MAC, leading to severely compromised defense against specific pathogens. Clinically, the most prominent and serious manifestation of C7 deficiency is a markedly increased susceptibility to invasive infections caused by Neisseria species—particularly Neisseria meningitidis, which causes conditions such as epidemic meningococcal meningitis and bacteremia. The underlying mechanism lies in the fact that the outer membrane structure of Neisseria renders these bacteria especially sensitive to complement-mediated lysis, and the MAC is the primary and most effective mechanism for host clearance of such pathogens. In individuals with C7 deficiency, complement activation can proceed to the formation of the C5b6 complex, but due to the absence of C7, this complex cannot be anchored to the bacterial surface to initiate subsequent MAC assembly, thereby completely disabling this crucial humoral immune defense.
Alternate Names for C7
C7; complement component 7; complement component C7;
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