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Complement C7 is a protein of the terminal complement system, recruited after C5b has associated with complement C6. It helps form the membrane-associated precursor to the membrane attack complex. It is one component of an ordered assembly, rather than a complete pore by itself.^[ref]

A 2016 cryo-electron microscopy study placed complement C7 between the early precursor and complement C8. Contacts on different parts of complement C7 connected it with both neighbors, explaining how one component can help propagate assembly. Its presence was also checked by mass spectrometry and functional evidence.^[ref]

An original supply model makes the sequential dependency tangible. Suppose eight competent early precursors are available but only five complement C7 molecules, and each completed intermediate requires one. At most five such intermediates can form. Supplying more complement C9 would not remove the shortage at this earlier step. The count is an upper bound under the stipulated model, not a predicted yield of functioning pores.

A useful functional comparison would reconstitute a defined set of components with and without complement C7, measuring both assembled material and membrane leakage. Detecting the protein, detecting a complex and detecting leakage answer different questions. A failed leakage assay alone would not identify which assembly step failed.

Complement C8 is a three-subunit protein complex participating in the terminal complement system. Its α, β and γ chains are distinct components, unlike three copies of one identical chain. It joins the C5b-containing assembly after complement C7 and before recruitment of complement C9.^[ref]

Structural analysis of the assembled membrane attack complex identified the α and β chains as two different parts of the pore architecture. The γ chain supplied a separate structural feature used to orient the complex in the reconstruction. The resulting arrangement helped explain the interface for the first complement C9 molecule.^[ref]

An original inventory example distinguishes subunits from complexes. A preparation with seven competent α chains, five β chains and nine γ chains can supply at most five complete αβγ units if each requires one of each and assembly is otherwise unrestricted. Adding all21 chains and dividing by three would give seven, incorrectly ignoring which subunit is limiting. Real assembly also depends on molecular competence and binding, so even five is only a count-based upper bound.

This distinction matters when interpreting a protein assay. An antibody recognizing one chain may detect that chain in incomplete or altered material as well as in a functional complement C8 complex. Establishing abundance of one subunit, correct assembly and membrane-damaging function requires different evidence.

Complement C9 is a terminal complement system protein that can assemble in multiple copies as part of the membrane attack complex. Recruitment follows the C5b–complement C6–complement C7–complement C8 assembly. The repeated copies contribute much of the membrane pore, while the initiating components remain part of the complex.^[ref]

A 2016 experiment assembled pores on liposomes and checked function through fluorescent-dye release. The complexes were isolated and imaged as frozen-hydrated particles by cryo-electron microscopy. The reconstructed complete pore contained18 complement C9 copies. That experimental end-product count is not a universal threshold for membrane leakage.^[ref]

For an original accounting model, suppose ten suitable initiators and126 competent complement C9 molecules are available, and stipulate18 copies per completed modeled product. The C9 supply permits at most $\lfloor 126/18\rfloor =7$ such products. This is a conditional inventory calculation, not a claim that the assay cannot detect leakage from incomplete assemblies.

The experiment links two levels of evidence. A structural reconstruction describes the arrangement of the isolated particles; dye release tests a membrane consequence in the chosen liposome preparation. Neither readout alone measures the concentration of freely circulating complement C9. Defining the molecular species and the functional endpoint is therefore part of a quantitative interpretation.

The membrane attack complex is an assembly of complement system proteins that can form an opening through a target lipid bilayer. Assembly begins with C5b and recruits complement C6, complement C7, complement C8 and multiple copies of complement C9. The resulting structure connects a soluble protein cascade to a physical change in membrane permeability.^[ref]

The scientific puzzle is how proteins carried in solution become a membrane-spanning assembly. A 2012 study determined the crystal structure of complement C6 and compared it with related structures. The authors proposed that coordinated conformational changes expose membrane-inserting regions as components join the growing complex. The directly measured C6 structure and the proposed assembly pathway are distinct levels of evidence.^[ref]

An illuminating comparison is between detecting complement proteins near a membrane and demonstrating a completed membrane-spanning structure. Recruitment is one step; assembly and insertion are further steps. Counting a precursor or one bound component alone therefore cannot establish that a fully formed membrane attack complex has changed the membrane's transport properties.