Concept
Complement system
Cascade of opsonization, inflammation, and lysis
High-yield clue
All three pathways converge on C3, and C5b-C9 assemble the membrane attack complex (MAC) that lyses cells.
Overview
A cascade of serum proteins that enhances host defense through three activation pathways (classical, lectin/MBL, and alternative) converging on C3. It bridges innate and adaptive immunity and is a frequent exam topic.
Classification
- Immunology concept
- Three activation pathways
- Enzyme cascade of serum proteins
Lab & identification clues
- C3b is the major opsonin (tags microbes for phagocytosis)
- C3a and C5a are anaphylatoxins; C5a is a chemoattractant
- MAC (C5b-C9) causes membrane lysis, key against Neisseria
Associations
- Terminal complement deficiency and Neisseria susceptibility vocabulary
- Encapsulated organisms evade complement/opsonization
- Classical pathway triggered by antibody-antigen complexes
Commonly confused with
- Classical vs alternative pathway triggers
- Opsonization vs MAC lysis
Your notes
Original microbiology concept summary. Sources checked: OpenStax Microbiology 2e, NCBI Bookshelf Medical Microbiology, and CDC/WHO topic pages where applicable; reviewed 2026-06. Educational only; no diagnosis, treatment selection, infection-control instructions, or specimen-handling guidance.