This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "complement_dysregulation#C5 Convertase Activation and Terminal Pathway Assembly"). The module defines the expected pathophysiology structure; conforming nodes in disorder files should include the corresponding cell types, biological processes, and causal edges, specialized to their disease context. Key disease-specific substitutions at the trigger node: atypical haemolytic uraemic syndrome substitutes germline loss of fluid-phase or membrane regulators (CFH, CFI, CD46) or gain-of-function C3/CFB variants; paroxysmal nocturnal haemoglobinuria substitutes somatic PIGA mutation with consequent absence of GPI-anchored CD55 and CD59; C3 glomerulopathy substitutes acquired convertase-stabilising autoantibodies. Anti-AChR myasthenia gravis is **not** a trigger-node conformer - it has no regulator lesion, and its autoantibody initiates the classical pathway - so it attaches at the terminal node, whose `GO:0001905` / `GO:0005579` binding its own curated node carries. Substitutions at the effector node follow the injured cell: erythrocyte (PNH), glomerular endothelium (aHUS, C3G), post-synaptic muscle membrane (MG). Complement component chemistry and the individual complement proteins are described in prose only; modules bind GO and CL terms only and do not use chemical (CHEBI) or protein term bindings.
Loss of Complement Regulatory Control
trigger
The complement alternative pathway is constitutively active at a low level and is restrained, rather than switched on, by a dedicated set of regulators acting both in the fluid phase (factor H, factor I) and on host membranes (CD46/MCP, CD55, CD59, thrombomodulin). Disease begins when that restraint is lost. The lesion may be a germline loss-of-function variant in a regulator, a gain-of-function variant in C3 or factor B that renders the convertase resistant to regulation, somatic loss of the GPI anchor that displays CD55 and CD59 on the cell surface, or an acquired autoantibody that stabilises the convertase or neutralises factor H. These are mechanistically distinct lesions with one shared consequence, and it is that shared consequence, not the lesion, that the rest of this chain models.
Downstream
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Unrestrained Alternative Pathway Amplification
Unrestrained Alternative Pathway Amplification
amplifier
Freed from regulation, the alternative pathway C3 convertase (C3bBb) runs as a positive-feedback amplification loop: each C3b generated recruits factor B to form further convertase, depositing more C3b on the surface. Because the loop is self-amplifying, a modest failure of restraint produces a large, sustained increase in surface-bound C3b rather than a proportionate one. This node is also the point of entry for classical- and lectin-pathway initiated disease, in which an immune complex or a carbohydrate ligand supplies the initiating C3b that the alternative pathway then amplifies.
Downstream
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C5 Convertase Activation and Terminal Pathway Assembly
C5 Convertase Activation and Terminal Pathway Assembly
central effector
Dense C3b deposition converts the C3 convertase into a C5 convertase, which cleaves C5 into C5a and C5b. C5a is a potent anaphylatoxin and neutrophil chemoattractant; C5b nucleates the sequential assembly of C6, C7, C8 and multiple C9 molecules into the lytic C5b-9 membrane attack complex in the target membrane. This is the disorder-agnostic, rate-limiting step of the module: every conforming disease funnels through it regardless of which regulator failed upstream, and it is the step at which terminal complement inhibitors act. It is therefore the key conformance target.
Downstream
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Complement-Mediated Cell Injury and Endothelial Activation