→ cleave C3 → C3a & C3b.

(Dr. Najeeb’s favorite table):

| Feature | MHC Class I | MHC Class II | |---------|-------------|---------------| | Expressed on | All nucleated cells | APCs (dendritic cells, macrophages, B cells) | | Presents antigen to | CD8+ T cells (cytotoxic) | CD4+ T cells (helper) | | Source of peptide | Endogenous (virus inside cell) | Exogenous (engulfed bacteria) | | Structure | α chain + β2-microglobulin | α & β chains |

| Subset | Induced by | Key cytokine | Function | |--------|-----------|--------------|----------| | Th1 | IL-12 | IFN-γ | Activates macrophages → kills intracellular bacteria (TB) | | Th2 | IL-4 | IL-4, IL-5 | Helps B cells, eosinophils → parasites, allergies | | Th17 | TGF-β + IL-6 | IL-17 | Recruits neutrophils → extracellular bacteria/fungi | | Treg | TGF-β | IL-10, TGF-β | Suppresses immune response (prevents autoimmunity) | | Tfh | IL-6, IL-21 | IL-21 | Helps B cells in germinal centers |