Which component is responsible for initiating the membrane attack complex?

The initiation of the membrane attack complex (MAC) in the complement system is primarily attributed to C5b. Upon activation of the complement cascade, specifically through the classical, lectin, or alternative pathways, complement proteins cleave to form various active components. C5 is cleaved into C5a and C5b, and it is the C5b fragment that plays a crucial role in assembling the MAC.

C5b serves as a platform for the sequential binding of further complement proteins, such as C6, C7, C8, and C9, in order to form the pore-like structure known as the membrane attack complex. This complex inserts into the membrane of pathogens, leading to cell lysis.

In contrast, while C4b, C3b, and C2a are important components in the complement activation process, they do not directly initiate the MAC. C4b and C2a are primarily involved in the formation of the C3 convertase in the classical pathway, and C3b is crucial for opsonization and also for the formation of the C5 convertase but does not form the MAC by itself. Therefore, C5b is the essential component that initiates the formation of the membrane attack complex