Complement membrane attack complex and upstream complement components (MAC)

The complement system is a sophisticated network of plasma proteins that serves as a primary arm of the innate immune system, facilitating the clearance of pathogens and damaged cells. The membrane attack complex (MAC), composed of components C5b, C6, C7, C8, and multiple C9 molecules, represents the terminal stage of the complement cascade, forming transmembrane channels that lead to the osmotic lysis of target cells (Merck Manuals, 2023). Upstream components, such as C1, C3, and C4, act as critical regulators and amplifiers, with C3 serving as a central convergence point for the classical, lectin, and alternative activation pathways (StatPearls, 2023). Dysregulation or overactivation of these components is a key driver in various rare and autoimmune diseases, including paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS), where the MAC causes unintended destruction of host red blood cells or endothelial tissue (NIH, 2022). Therapeutic strategies often target C5 to prevent MAC assembly while sparing upstream opsonization, or target C3 to provide more comprehensive pathway blockade (Nature Reviews Drug Discovery, 2021). However, because the complement system is vital for defending against certain bacteria, pharmacological inhibition necessitates strict monitoring and prophylactic measures against meningococcal infections (FDA Soliris Label, 2020).

Other names

Complement cascadeTerminal complement complexTCCC5b-9 complexComplement system

Inhibition of C5 cleavage to prevent the formation of C5a and the membrane attack complex (C5b-9); inhibition of C3 to block all three activation pathways; inhibition of C1s to selectively block the classical pathway; or inhibition of Factor B/D to block the alternative pathway.