Introduction and General Properties of the Complement System
The complement system (CS) is a critical component of the innate immune system and a bridge to adaptive immunity. It is not a single entity but a tightly regulated cascade comprising over 30 distinct proteins, including soluble plasma proteins (zymogens), cell-surface receptors, and regulatory factors. Its name, “complement,” derives from its historical discovery in the late 19th century as a heat-labile component of serum that ‘complemented’ or assisted the action of antibodies in killing bacteria and promoting immune responses. Collectively, these proteins make up approximately 5% of the total plasma proteins, circulating in an inactive zymogen form until they encounter a biological surface that signals danger, such as a pathogen or an altered host cell like an apoptotic body. The subsequent activation proceeds as a finely tuned enzyme cascade, amplifying the initial signal and leading to a series of coordinated biological outcomes that are essential for host defense and the maintenance of homeostasis.
Historical Overview and Early Nomenclature
The nomenclature of the complement system is complex, reflecting its historical, piece-meal discovery, rather than the sequential order of its reactions. Initially, the components of what is now known as the Classical Pathway were numbered in the order of their isolation and discovery, leading to the designation of components C1 through C9. The original convention, established in 1963, used the letter “C” followed by a number (C1, C2, C3, etc.) for the inactive forms. When a component was cleaved into active fragments, the larger, typically surface-binding fragment was traditionally designated with a prime symbol (e.g., C’3), which was later standardized to a lowercase letter. As the reaction sequence was deciphered, it was found that the activation order of the Classical Pathway components is C1, C4, C2, C3, and then C5 through C9. The initial components were not only numbered out of sequence but C1 was quickly found to be a complex itself, composed of three subcomponents: C1q, C1r, and C1s.
A crucial rule in the nomenclature relates to the active fragments generated by proteolytic cleavage. Generally, when a complement component is cleaved, the resulting fragments are designated with the component’s number followed by a lowercase letter. The larger fragment, which usually binds to the target surface and continues the cascade, is denoted with the suffix “b” (e.g., C3b, C5b), while the smaller, soluble fragment, which often acts as a chemoattractant or inflammatory mediator, is denoted with the suffix “a” (e.g., C3a, C5a). However, a historical exception that continues to cause confusion is C2. When C2 is cleaved, the larger, active fragment that forms the C3 convertase complex was initially and is still sometimes designated C2a, while the smaller, inactive fragment is C2b. Modern efforts promote consistency by attempting to rename the larger fragment C2b, but C2a remains prevalent in older literature and some current texts. Enzymatically active complexes are often named by listing their constituent fragments, such as C4b2a (Classical C3 convertase) or C3bBb (Alternative C3 convertase).
The Three Complement Activation Pathways
Complement activation proceeds through three distinct yet convergent pathways, all of which lead to the central step of C3 cleavage. These are the Classical Pathway (CP), the Lectin Pathway (LP), and the Alternative Pathway (AP).
The Classical Pathway is typically initiated by the binding of the C1 complex (C1q, C1r, C1s) to the Fc region of antibody-antigen complexes (specifically IgM or certain subclasses of IgG). C1q, the recognition subcomponent, must bind to at least two antibody molecules to trigger the activation of the C1r and C1s proteases. Active C1s then cleaves C4 and C2, leading to the formation of the C4b2a complex, which is the Classical Pathway C3 convertase.
The Lectin Pathway is antibody-independent and operates on a mechanism structurally homologous to the Classical Pathway. It is initiated by the binding of a pattern-recognition molecule, such as Mannose-Binding Lectin (MBL) or a Ficolin, to carbohydrate structures (PAMPs, or Pathogen-Associated Molecular Patterns) on the surface of microbes. Upon binding, MBL or Ficolin activates associated serine proteases, known as MBL-Associated Serine Proteases (MASP-1 and MASP-2). Active MASP-2 then cleaves C4 and C2, leading to the formation of the same C4b2a C3 convertase as the Classical Pathway.
The Alternative Pathway acts as a constant surveillance system and is unique because it can be initiated spontaneously in the fluid phase, known as “tick-over,” without requiring a specific recognition molecule like an antibody or MBL. Spontaneous hydrolysis of C3 forms C3(H2O), which then binds to Factor B (FB). This complex is cleaved by Factor D (FD) to form the initial C3 convertase, C3(H2O)Bb. This convertase is highly unstable unless it lands on a non-host (activating) surface, where the C3b fragment it generates can be stabilized by a plasma protein called Properdin (P). This forms the stable C3bBbP complex, the Alternative Pathway C3 convertase. The components of the Alternative Pathway were designated by letters (Factor B, Factor D, Factor H, Factor I, and Properdin) to distinguish them from the initial C-numbered proteins.
Convergence and Effector Functions
All three pathways converge at the cleavage of C3. Both C3 convertase types (C4b2a and C3bBbP) proteolytically cleave thousands of C3 molecules into the large C3b fragment and the small C3a fragment. C3b is the main effector molecule of the system; it deposits covalently on the pathogen surface, marking it for destruction, a process called Opsonization. The small, soluble fragment C3a diffuses away and functions as an Anaphylatoxin, causing degranulation of mast cells and basophils, which releases inflammatory mediators and promotes Inflammation and Chemotaxis (recruiting phagocytes).
C3b also combines with its respective C3 convertase to form a C5 Convertase (C4b2a3b in the Classical/Lectin pathway; C3bBbC3b in the Alternative pathway). The C5 convertase cleaves C5 into C5a (a potent anaphylatoxin and chemoattractant) and C5b. C5b then initiates the terminal pathway, which culminates in the formation of the Membrane Attack Complex (MAC). The MAC is a lytic pore formed by the sequential assembly of C5b, C6, C7, C8, and multiple copies of C9. This complex inserts into the pathogen’s cell membrane, creating an osmotic pore that causes the pathogen to swell and lyse, thereby leading to Cell Lysis.
Regulation and Comprehensive Significance
Given the destructive potential of the MAC and the inflammatory nature of the anaphylatoxins, the complement cascade must be tightly regulated to protect host tissues. Regulation occurs at multiple levels and involves a host of specialized regulatory proteins, which are also included in the complement nomenclature. These regulators act as inhibitors, cofactors for inactivation, and decay accelerators. For example, C1 Inhibitor (C1-INH) controls the Classical and Lectin pathways by inhibiting C1r and C1s proteases and MASP-2. Factor H (FH) and Factor I (FI) are critical soluble regulators of the Alternative Pathway. FH binds to host cell surfaces, prevents C3b from associating with Factor B, and acts as a cofactor for FI, which cleaves and inactivates C3b into iC3b and C3dg, thus preventing further amplification. Membrane-bound regulators like Decay Accelerating Factor (DAF/CD55), Membrane Cofactor Protein (MCP/CD46), and Protectin (CD59) are strategically placed on host cells to block complement activation at the C3 convertase and MAC formation steps, ensuring that the system’s destructive force is restricted to foreign surfaces. The significance of the complement system extends beyond pathogen defense to include the clearance of immune complexes and apoptotic cells, linking it directly to the prevention of autoimmune diseases.
Summary of Key Nomenclature and Functions
The complement nomenclature, while historically inconsistent, provides precise terms for a highly organized system: C-numbers for Classical pathway and terminal components (C1-C9), F-letters for Alternative pathway factors (FB, FD, FH, FI, P), and MASP/MBL for Lectin pathway initiators. Fragments are generally designated ‘a’ (smaller, soluble, inflammatory) or ‘b’ (larger, surface-bound, functional), with C2a being the primary exception that is often still used for the larger fragment. Active complexes are identified by the components they contain (e.g., C4b2a is C3 convertase). The system’s properties are summarized by its four main functions: Opsonization (C3b tagging), Lysis (MAC formation), Inflammation (C3a/C5a), and Clearance (of cellular debris and immune complexes). The tight regulation by proteins like C1-INH and Factor H underscores the essential balance required for this powerful immune cascade to protect, rather than harm, the host.