Which complement proteins assemble into the membrane-attack complex that kills bacteria by disrupting their osmotic balance?

1. The complement proteins are found constitutively in blood plasma and act as a cascade, each protein altering and fragmenting the next. 2. The splitting of the C3 protein is the step common to both the alternate and the classical pathway. 3. The larger fragment C3b binds the surface of the pathogen and opsonizes it, while the smaller C3a diffuses outward and attracts phagocytes. 4. Surface-bound C3b then activates the rest of the cascade, and the last five proteins, C5 to C9, form the membrane-attack complex. 5. That complex can kill certain pathogens by disrupting their osmotic balance and is especially effective against a broad range of bacteria. 6. C1 reacts with antibody at the start of the classical pathway, so the low numbers are the trap. 7. Factor P, factor B and factor D act on C3 in the alternate pathway rather than building the pore. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 21 "The Lymphatic and Immune System", section 21.2 Barrier Defenses and the Innate Immune Response_