The Innate Immune System
High-Yield Summary
- Noncellular barriers (skin, respiratory tract mucus/cilia/lysozyme, GI stomach acid + gut flora) stop pathogens before infection starts.
- Complement system: classical pathway (antibody-triggered), alternative pathway (direct pathogen activation), lectin pathway (mannose-binding lectin) — all opsonize pathogens, recruit immune cells, and assemble the membrane attack complex (MAC).
- Interferons are secreted by virus-infected cells to warn neighbors, degrade viral RNA, boost antigen presentation, and activate NK cells/macrophages.
- MHC I (endogenous, all nucleated cells, read by CD8+ T cells) vs. MHC II (exogenous, professional APCs only, read by CD4+ T cells).
- Dendritic cells = most potent APCs. Toll-like receptors let macrophages/dendritic cells detect pathogen patterns directly and release cytokines.
- NK cells kill infected/cancerous cells (especially low/absent MHC I) WITHOUT needing prior antigen exposure — unlike cytotoxic T cells.
Key Terms
- Defensins
- Antimicrobial peptides produced by skin that disrupt microbial membranes.
- Lysozyme
- Enzyme in the respiratory tract that breaks down peptidoglycan in bacterial cell walls.
- Opsonization
- Marking a pathogen (by complement or antibodies) to make it easier for phagocytes to engulf.
- Membrane attack complex (MAC)
- Complement protein assembly that punctures a pathogen's membrane, causing lysis.
- Toll-like receptors (TLRs)
- Pattern recognition receptors on macrophages/dendritic cells that detect common pathogen molecular patterns and trigger cytokine release.
MHC Class I vs. MHC Class II
| MHC Class I | MHC Class II |
|---|---|
| Endogenous pathway (antigens made inside the cell) | Exogenous pathway (antigens from outside the cell) |
| Present on ALL nucleated cells | Present only on professional APCs (macrophages, dendritic cells, some B cells) |
| Read by CD8+ cytotoxic T cells | Read by CD4+ helper T cells |
Cellular Defenses
- Macrophages
- Phagocytic cells + antigen-presenting cells (APCs); display engulfed antigens on MHC molecules.
- Dendritic cells
- Most potent APCs; capture antigens in peripheral tissues and migrate to lymph nodes to activate T cells.
- Mast cells
- Release histamine and heparin, increasing vascular permeability; central to allergic reactions.
- Natural killer (NK) cells
- Lymphocytes of innate immunity; kill virus-infected/cancerous cells lacking MHC I, no prior exposure needed.
Common MCAT Trap
- NK cells vs. cytotoxic T cells: NK cells need NO prior antigen exposure and target cells with reduced/absent MHC I; cytotoxic T cells DO need prior exposure and recognize abnormal antigen displayed ON MHC I.
- MHC I is on all nucleated cells (not just APCs); MHC II is restricted to professional APCs. Mixing these up is a classic wrong-answer trap.
- The complement system's classical pathway needs antibodies (bridges innate/adaptive) — the alternative and lectin pathways don't.
Quick Recall
What 3 things can activated complement proteins do?
Which MHC class is read by CD8+ T cells, and where do its antigens come from?
What makes NK cells different from cytotoxic T cells?
What do toll-like receptors do?