C2 (complement component 2) is the catalytic component of the classical complement pathway's C3 and C5 convertase complexes. Following complement activation, C2 is recruited to pathogen surfaces via C4b opsonin to form the C3 convertase complex, or alongside C3b and C4b to form the C5 convertase 12. As part of the C3 convertase, C2 cleaves C3 into the anaphylatoxin C3a and opsonin C3b, amplifying complement activation 32. Within the C5 convertase, C2 activates C5 into anaphylatoxin C5a and C5b, initiating membrane attack complex formation 12. The 18 kb C2 gene contains 18 exons encoding a serine protease domain and von Willebrand factor type A-like domain 4. Alternative splicing generates both secreted and intracellular C2 protein isoforms, with the secreted form predominating at a 2:1 ratio in liver, macrophages, and fibroblasts 5. C2 deficiency causes immune susceptibility, while C2 variants associate with age-related macular degeneration, highlighting the protein's critical role in immune defense and tissue homeostasis.