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The complement system activates through three distinct pathways: the classical pathway, triggered when antibodies bind to antigens on pathogens; the lectin pathway, activated by mannose-binding lectin proteins attaching to pathogens; and the alternative pathway, which maintains continuous low-level activity when complement proteins interact with specific molecules on pathogen surfaces. All three pathways eventually converge to cleave C3.

The complement system marks pathogens through opsonization, a process where complement proteins, particularly C3b, coat the pathogen surface. This coating makes pathogens more recognizable to phagocytes, which engulf and destroy the marked invaders. Opsonization is one of three primary defense mechanisms of the complement system, alongside inflammation and cell lysis.

C3a is a fragment produced when complement protein C3 is cleaved during the cascade. It plays a crucial role in promoting inflammation by attracting immune cells to the infection site and enhancing vascular permeability. This inflammatory response helps mobilize the body's defenses to combat the pathogen more effectively.

The membrane attack complex (MAC) forms when several complement proteins assemble on the pathogen surface, creating pores in the cell membrane. These pores compromise membrane integrity, leading to cell lysis and pathogen destruction. C3b contributes to MAC formation, making it essential for this direct killing mechanism of the complement system.

The classical pathway is the most rapid and effective mode of complement activation because it is triggered directly when the C1 complex binds to antibodies already attached to antigens on pathogen surfaces. This antibody-dependent mechanism provides immediate recognition and swift activation of the cascade, enabling faster immune response compared to the antibody-independent lectin and alternative pathways.

C3b is the larger fragment produced when C3 is cleaved and serves as a central hub in the complement cascade. It binds to pathogen surfaces for opsonization, activates other complement proteins to continue the cascade, and contributes to membrane attack complex formation. C3b essentially amplifies and perpetuates the complement response across all three activation pathways.

Approximately 20 complement plasma proteins, labeled C1 through C9 along with regulatory proteins, typically circulate in an inactive state throughout the blood. This inactive circulation allows them to remain available for rapid deployment when needed. Upon exposure to specific activation signals from pathogens, these proteins are triggered to initiate the cascade and execute their defensive functions.