Complement system classical pathway

The **complement system classical pathway** is one of three major activation pathways of the complement system, a crucial part of innate and adaptive immunity. It is initiated primarily by antigen-antibody complexes (usually IgG or IgM) that bind to the C1q component of the C1 complex, leading to a cascade involving C1r, C1s, C4, and C2, which forms the C3 convertase (C4b2a)[3][4][5][8]. The pathway amplifies pathogen clearance by promoting opsonization, phagocytosis, inflammation (via anaphylatoxins C3a and C5a), and direct cell lysis through the membrane attack complex (MAC)[3][4][8]. While the pathway itself is not a single molecular target, but a sequence of serum proteins and reactions, therapeutic intervention is achieved by inhibiting specific proteins (such as C1s with sutimlimab) to modulate classical pathway activity in diseases like autoimmune hemolytic anemia or hereditary angioedema[1][7]. Dysfunction or overactivation of the pathway is linked to conditions such as autoimmune disease, infection, and inflammatory syndromes[7][4]. Because "complement system classical pathway" refers to a biological process, not a single molecule/receptor, it is not suitable as a standard therapeutic target, though proteins within the pathway can serve as valid targets.

Other names

Complement classical pathwayClassical complement pathwayComplement activation classical pathway

Inhibition of complement protein activity (e.g., C1, C1s), blockade of membrane attack complex formation, and suppression of antibody-mediated complement activation.