Complement C7

The complement system functions mainly depending on the formation of membrane attack complex (MAC) on the surface of pathogen cell membranes, leading to cell lysis and death. Similar to complement C5b and C6, complement C7 (C7) is the third essential constituent of the MAC. Human C7 is a single-chain elongated glycoprotein composed of 843 amino acids, containing nine domains (a MAC perforin domain and eight cysteine-rich modules). The concentration of C7 in normal human plasma is usually between 50-90 μg/mL, which is mainly synthesized by monocytes, not hepatocytes.

In the process of MAC formation, C7 binds to the C5b-6 complex, after which the C5bC6C7 (C5b-7) complex exposes the membrane-binding region and partially inserts into the bilipid layer of the target cell. C5b-7 can bind to a C8, allowing the complex to be more firmly inserted into the membrane. It has been reported that C7 could bind to endothelial cells and exert an anti-inflammatory effect, and potentially target plasmin activity to complement deposition sites by binding to plasminogen. C7 deficiency is associated with increased susceptibility to neisserial infections (such as meningococcal infections) and immunodeficiency due to a late component of complement deficiency.

Fig. 1 C7 is the significant component of MAC.¹

Reference

1. From Wikipedia: By SLiva2016 - Own work, CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:Membrane_Attack_Complex_(Terminal_Complement_Complex_C5b-9).png