Figure 3.

Schematic overview of the complement system highlighting two of the main activation pathways (classical vs. alternative). Previous studies have shown that classical pathway activation is initiated by antibody-liposome binding, pegylated liposomes, and anionic liposomes. The alternative pathway can be initiated by cationic liposomes, liposomes containing more than 40 mol% cholesterol, and liposomes larger than 200 nm in diameter. During the complement cascade, C3b opsonin covalently binds to the surface of the liposome, marking it for removal by the MPS. The released anaphylatoxins (C3a, C4a, and C5a) prompt the activation of leukocytes.