The innate immune system is the parts of immune function that
- Do not require prior exposure to a pathogen to be effective
- Are active from birth
- Typically react to highly conserved molecular features of pathogens
Physical barriers
- Skin
- Mucus
- Mucocilliary escalator
Chemical barriers
- Lysozyme in saliva hydrolyses peptidoglycan \(\to\) bacterial lysis
- Low pH in stomach acid \(\to\) bacterial death
Complement
- Set of proteins mostly produced in liver
- Activation \(\to\) C3 amplification loop \(\to\) endpoints
- Activation triggers: classical (Ig-epitope complex), alternative (bacterial peptigoglycan), lectin (activated by mannin on bacterial and fungal cell wall)
- Classical activation: Ig-epitope complex \(\to\) C1 \(\to\) C4 \(\to\) C2 \(\to\) C3
- Alternative: Peptidoglycan \(\to\) C3
- Lectin: Mannins \(\to\) C4 \(\to\) C2 \(\to\) C3
- C3 amplification loop: positive feedback to lyse lots of C3, then rapidly activates downstream components
- Endpoints: molecular attack complex (C5,6,7,8,9; forms pore in target cell \(\to\) lysis), opsonins, chemotaxis
Innate immune cells
- Neutrophils - phagocytes released from bone marrow that phagocytose opsonized targets
- Macrophages - native phagocytes that recognize targets with toll-like receptors, or opsonized targets
- NK cells - lyse cells that lack MHC1 (prevents immune avoidance)
- Mast cells - degranulate \(\to\) attract other immune cells
Nutritional immunity
- IL6 \(\to \ \uparrow\)hepcidine \(\to \ \downarrow\) ferroportin \(\to\) iron sequestration in macrophages, denying iron to pathogen