photo.jpg

### Differences between the classical, alternative and lectin pathways | Pathway | Classical | Alternative | Lectin | |---|---|---|---| | Type of immunity | Acquired | Innate | Innate | | Initiated by | Antigen-antibody complex | Microbial surface molecules (alternative pathway) | Mannose residue o...

### Differences between the classical, alternative and lectin pathways | Pathway | Classical | Alternative | Lectin | |---|---|---|---| | Type of immunity | Acquired | Innate | Innate | | Initiated by | Antigen-antibody complex | Microbial surface molecules (alternative pathway) | Mannose residue on the surface of microorganisms (lectin pathway) | | Activation of complement | All components starting with C1 | Bypasses C1, C4 and C2, starting with C3 | All components except C1 | ### Biological activities of the complement * **Cell lysis:** This occurs when the antigen is on the cell wall of bacteria or cells e.g. RBCs, graft cells, or tumor cells. Insertion of the membrane attack complex into the cell membrane results in its disruption and the entry of water and electrolytes into the cell leading to lysis. * **Opsonization:** Cells and antigen-antibody complexes are phagocytosed much better when combined with C3b (acts as an opsonin). * **Inflammatory reactions:** * C3a activation and attracts neutrophils and macrophages to the site of inflammation i.e. "chemotactic". * C3a, C4a and C5a are "anaphylatoxins" i.e. cause degranulation of mast cells and release of histamine and other inflammatory mediators, thus promoting the inflammatory reaction.