Complement system alternative pathway

The alternative pathway of the complement system is one of three main activation pathways of the innate immune complement cascade. It operates independently of antibodies, providing rapid immune defense by spontaneous low-level hydrolysis of C3, which, upon activation and amplification, leads to opsonization of pathogens, recruitment of immune cells (via anaphylatoxins like C3a and C5a), and direct microbial lysis through formation of the membrane attack complex. This pathway acts as both a recognition and amplification system, with strict regulatory controls to prevent host tissue damage. Dysregulated alternative pathway activity is involved in multiple inflammatory and autoimmune diseases. It is a validated drug target, with both approved and investigational inhibitors specifically targeting the pathway at various points to reduce pathological complement activation.

Other names

alternative complement pathwayalternative pathway of complementcomplement alternative pathwayAP

Inhibition of complement protein cleavage (e.g., C5 inhibition prevents membrane attack complex formation). Inhibition of alternative pathway specific factors (e.g., Factor B or Factor D inhibition prevents formation of the AP C3 convertase, blocking pathway activation and downstream effects).