Complement component 3b (C3b) and Complement component 4b (C4b) (C3b/C4b)

Complement components C3b and C4b are essential activation fragments of the complement system, serving as central mediators of the innate immune response. C4b is generated via the classical or lectin pathways and forms the C3 convertase (C4b2a), while C3b is produced by all three pathways and is critical for the formation of C5 convertases and the alternative pathway amplification loop [Source: Janeway's Immunobiology]. Both fragments function as potent opsonins, covalently attaching to the surface of pathogens or damaged cells to promote their recognition and ingestion by phagocytes via complement receptors [Source: Ricklin et al., 2010, Nature Immunology]. Dysregulation of C3b and C4b is a hallmark of several diseases, including paroxysmal nocturnal hemoglobinuria (PNH), where uncontrolled C3b-mediated opsonization leads to extravascular hemolysis, and C3 glomerulopathy, characterized by C3 fragment deposition in the kidneys [Source: Mastellos et al., 2019, Trends in Immunology]. Therapeutic targeting of these components aims to modulate the complement cascade to prevent tissue injury. For example, pegcetacoplan binds to C3 and C3b to inhibit convertase activity, while investigational agents like mirococept and soluble CR1 mimic natural regulators to accelerate the decay of convertases and promote the degradation of C3b and C4b [Source: Hillmen et al., 2021, NEJM; Smith et al., 2019, Frontiers in Immunology].

Other names

C3bC4bComplement C3bComplement C4bOpsonin C3bOpsonin C4bActivated Complement Component 3Activated Complement Component 4

The mechanism of action for drugs targeting C3b and C4b involves the inhibition of C3 and C5 convertase assembly and activity. By binding to these fragments, therapeutic agents prevent the further cleavage of C3 and C5, thereby blocking the production of proinflammatory anaphylatoxins (C3a, C5a) and the formation of the membrane attack complex (C5b-9). Some agents also function as cofactors for Factor I, facilitating the proteolytic degradation of C3b and C4b into inactive forms (iC3b, C3dg, C4d), which reduces opsonization and downstream signaling [Source: Ricklin et al., 2018, Nature Reviews Drug Discovery; Mastellos et al., 2019, Trends in Immunology].