12.1 Innate Immune Barriers, Phagocytosis, Cellular Pattern Recognition, and Complement System

12.1 Innate Immune Barriers, Phagocytosis, Cellular Pattern Recognition, and Complement System

The immune system is divided into two cooperative arms: innate immunity and adaptive immunity. Innate immunity serves as the body's first line of defense, providing immediate, non-specific protection against invading pathogens. Unlike adaptive immunity, innate immune responses do not require prior exposure to a pathogen, operate with germline-encoded recognition receptors, and do not generate long-term antigen-specific immunological memory.

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Innate vs. Adaptive Immunity

The key distinguishing features between innate and adaptive immune responses are summarized below:

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Physical and Chemical Barriers

Innate protection begins at the anatomic surfaces, which combine physical exclusion with antimicrobial biochemistry:

• Skin (Cutaneous Barrier): The stratified squamous epithelium of the skin, topped by the keratinized stratum corneum and secured by tight junctions, forms an impenetrable mechanical barrier. Sebaceous glands secrete free fatty acids and lactic acid, maintaining an acidic surface pH (4.5–5.5) that inhibits microbial colonization.
• Lysozyme: An enzyme present in high concentrations in tears, saliva, nasal secretions, and neutrophil granules. Lysozyme cleaves the $\beta(1\rightarrow 4)$ glycosidic bond between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in bacterial peptidoglycan cell walls, causing osmotic lysis (particularly of Gram-positive bacteria).
• Gastric Acid: Gastric parietal cells secrete hydrochloric acid (HCl), lowering stomach pH to 1.0–2.0. This extreme acidity destroys the vast majority of ingested vegetative bacteria, fungi, and enveloped viruses.
• Mucus and Mucociliary Escalator: Mucosa-lined tracts (respiratory, gastrointestinal, urogenital) secrete mucin glycoproteins that trap particulate matter and microbes. In the upper respiratory tract, ciliated epithelial cells push trapped particles upward toward the pharynx (mucociliary escalator) for clearance by swallowing or coughing.

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Pattern Recognition Receptors (PRRs)

Myeloid cells (macrophages, dendritic cells, neutrophils) detect pathogens using germline-encoded Pattern Recognition Receptors (PRRs) that recognize conserved Pathogen-Associated Molecular Patterns (PAMPs) or host-derived Damage-Associated Molecular Patterns (DAMPs).

Toll-Like Receptors (TLRs)

TLRs are transmembrane glycoproteins located either on the cell surface or within endosomal membranes:

• Cell Surface TLRs:
  • TLR4: Recognizes lipopolysaccharide (LPS) of Gram-negative bacterial outer membranes. TLR4 dimerization requires accessory proteins LBP (LPS-binding protein), CD14, and MD-2, triggering downstream NF-$\kappa$B activation and pro-inflammatory cytokine production (TNF-$\alpha$, IL-1, IL-6).
  • TLR2 (with TLR1 or TLR6): Recognizes bacterial lipopeptides, peptidoglycan, and lipoteichoic acid (Gram-positive bacteria).
  • TLR5: Recognizes bacterial flagellin.
• Endosomal TLRs:
  • TLR3: Recognizes viral double-stranded RNA (dsRNA).
  • TLR7 / TLR8: Recognizes viral single-stranded RNA (ssRNA).
  • TLR9: Recognizes unmethylated CpG DNA motifs prevalent in bacterial and viral genomes.

NOD-Like Receptors (NLRs) and the Inflammasome

NOD-like receptors are cytosolic PRRs. NOD1 and NOD2 sense peptidoglycan fragments in the cytoplasm. NLRP3 senses intracellular stress (e.g., potassium efflux, uric acid crystals, reactive oxygen species), forming the NLRP3 inflammasome. Assembly of the inflammasome activates caspase-1, which cleaves inactive precursors pro-IL-1$\beta$ and pro-IL-18 into mature, secreted IL-1$\beta$ and IL-18, driving systemic inflammation and fever.

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Phagocytosis and the Respiratory Burst

Phagocytosis is the process by which professional phagocytes (neutrophils and macrophages) engulf and digest extracellular particles and pathogens.

Steps of Phagocytosis

1. Chemotaxis: Phagocytes migrate along a chemical gradient toward chemoattractants, including C5a, leukotriene B4 ($LTB_4$), interleukin-8 (IL-8), and bacterial N-formylmethionine peptides (fMLP).
2. Opsonization: Pathogens are coated with host opsonins—primarily IgG (Fc region) and complement fragment C3b (or $iC3b$). Opsonization drastically enhances phagocytosis by binding phagocyte $Fc\gamma R$ receptors and complement receptor 1 (CR1).
3. Engulfment: Pseudopods extend around the opsonized particle, fusing to form an internal membrane-bound vesicle called a phagosome.
4. Phagolysosome Fusion: The phagosome fuses with cytoplasmic lysosomes, exposing the pathogen to low pH, defensins, acid hydrolases, and lysosomal enzymes.
5. Oxidative (Respiratory) Burst: Phagocytes generate potent Reactive Oxygen Species (ROS) to destroy engulfed microbes:
   • NADPH Oxidase: Assembly of the membrane-bound NADPH oxidase complex transfers electrons from NADPH to molecular oxygen ($O_2$), generating superoxide anion ($O_2^{\bullet-}$): $2O_2 + \text{NADPH} \xrightarrow{\text{NADPH Oxidase}} 2O_2^{\bullet-} + \text{NADP}^+ + H^+$
   • Superoxide Dismutase (SOD): Converts superoxide to hydrogen peroxide ($H_2O_2$): $2O_2^{\bullet-} + 2H^+ \xrightarrow{\text{SOD}} H_2O_2 + O_2$
   • Myeloperoxidase (MPO): Contained in azurophilic granules, MPO uses $H_2O_2$ and chloride ions ($Cl^-$) to synthesize hypochlorous acid ($HOCl$, active ingredient in bleach): $H_2O_2 + Cl^- + H^+ \xrightarrow{\text{MPO}} HOCl + H_2O$

   O2  ---(NADPH Oxidase)--->  O2•-  ---(SOD)--->  H2O2  ---(MPO + Cl-)--->  HOCl (Microbicidal Bleach)

Chronic Granulomatous Disease (CGD)

CGD is an inherited immunodeficiency (most commonly X-linked recessive due to a mutation in the $gp91^{phox}$ subunit of NADPH oxidase). Phagocytes can engulf bacteria but cannot produce the oxidative burst ($O_2^{\bullet-}$ and $H_2O_2$ generation is impaired).

• Diagnostic Testing:
  • Nitroblue Tetrazolium (NBT) Test: Normal phagocytes reduce clear NBT dye into dark blue formazan precipitation; CGD phagocytes remain clear (negative test).
  • Dihydrorhodamine (DHR) Flow Cytometry: Measures rhodamine 123 fluorescence upon oxidation; CGD shows absent green fluorescence.
• Microbial Susceptibility: Patients suffer recurrent infections with catalase-positive organisms (Staphylococcus aureus, Pseudomonas cepacia/Burkholderia, Serratia marcescens, Aspergillus species, Nocardia). Catalase-positive microbes produce catalase, which degrades their own endogenous $H_2O_2$. Without bacterial $H_2O_2$, CGD phagocytes (lacking endogenous $H_2O_2$) cannot utilize MPO to produce $HOCl$. Conversely, catalase-negative organisms (e.g., Streptococcus pneumoniae) produce $H_2O_2$ that CGD phagocytes can "borrow" to synthesize $HOCl$ via MPO, allowing successful killing.

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The Complement System

The complement system comprises over 30 plasma proteins synthesized primarily by the liver that circulate as inactive zymogens. Complement activation occurs through three distinct pathways that converge at the formation of C3 convertase.

[Classical Pathway]       [Lectin Pathway]         [Alternative Pathway]
IgG/IgM + Ag -> C1qrs      MBL / Ficolins           Spontaneous C3 hydrolysis
        |                         |                          |
   Cleaves C4 & C2           Cleaves C4 & C2           Binds Factor B & D
        \                         /                          |
     Classical/Lectin C3 Convertase (C4b2a)      Alternative C3 Convertase (C3bBb)
                        \                         /
                         \                       /
                          --> [ C3 CONVERTASE ] <--
                                      |
                               Cleaves C3 into
                                    /   
                              C3a  /     \  C3b (Opsonization)
                                  /       
                                 v         v
                         [ C5 CONVERTASE (C4b2a3b / C3bBb3b) ]
                                      |
                               Cleaves C5 into
                                    /   
                              C5a  /     \  C5b
                         (Anaphylatoxin)  | + C6, C7, C8, C9
                                          v
                               [ Membrane Attack Complex ]
                                    (MAC: C5b-C9)
                                          |
                                    Cell Lysis

1. Activation Pathways

• Classical Pathway: Activated by antigen-antibody complexes. The C1 complex ($C1q, C1r, C1s$) binds the Fc region of IgM (pentameric structure makes a single IgM highly efficient) or multiple IgG molecules (IgG3 > IgG1 > IgG2). Activated $C1s$ cleaves $C4$ into $C4a$ and $C4b$, and $C2$ into $C2a$ and $C2b$. The active complex $C4b2a$ is the Classical C3 Convertase.
• Alternative Pathway: Activated spontaneously without antibodies. Slow, continuous hydrolysis of plasma C3 ("C3 tickover") yields $C3(H_2O)$. $C3(H_2O)$ binds Factor B, which is cleaved by Factor D to form the initial soluble convertase. Deposited $C3b$ on microbial cell surfaces (LPS, zymosan) binds Factor B and Factor D to form $C3bBb$, the Alternative C3 Convertase. This complex is stabilized by properdin (Factor P).
• Lectin Pathway: Activated independently of antibodies when Mannose-Binding Lectin (MBL) or ficolins bind terminal mannose or N-acetylglucosamine residues on microbial surfaces. MBL-Associated Serine Proteases (MASP-1 and MASP-2) cleave C4 and C2 to form $C4b2a$ (identical to the Classical C3 convertase).

2. Common Pathway, Effector Functions, and MAC

• C3 Cleavage: Both C3 convertases ($C4b2a$ or $C3bBb$) cleave C3 into C3a (released into fluid phase) and C3b (covalently attaches to pathogen surfaces).
• C5 Convertase: Attachment of C3b to C3 convertase forms C5 convertase ($C4b2a3b$ for classical/lectin; $C3bBb3b$ for alternative), which cleaves C5 into C5a and C5b.
• Membrane Attack Complex (MAC): C5b sequentially binds C6, C7, C8, and 10–16 C9 molecules, forming the hydrophobic pore C5b-C9. MAC insertion into target cell membranes disrupts osmotic equilibrium, inducing cell lysis. MAC-mediated lysis is essential for killing thin-walled Gram-negative bacteria, particularly Neisseria meningitidis and Neisseria gonorrhoeae. Patients with terminal complement deficiencies (C5–C9) suffer recurrent invasive Neisseria infections.
• Opsonization: C3b and its cleavage product $iC3b$ act as major opsonins. $C3b$ binds Complement Receptor 1 (CR1) on phagocytes and erythrocytes. Erythrocyte CR1 binds C3b-coated circulating immune complexes, transporting them to the spleen and liver for macrophage clearance.
• Anaphylatoxins ($C3a, C4a, C5a$): $C3a$ and $C5a$ induce mast cell degranulation, releasing histamine that increases vascular permeability and smooth muscle contraction. C5a is also a potent neutrophil chemoattractant and activator.

3. Complement Regulation and Associated Diseases

• C1 Esterase Inhibitor (C1-INH): Inactivates $C1r, C1s$, and MASPs, and inhibits kallikrein in the kinin cascade. C1-INH Deficiency leads to Hereditary Angioedema, characterized by uncontrolled kallikrein activity and excessive bradykinin generation. Patients experience recurrent, non-pitting subepithelial edema of the skin, GI tract (severe abdominal pain), and larynx (life-threatening airway obstruction) without urticaria or pruritus. (Note: ACE inhibitors are strictly contraindicated).
• GPI-Anchored Regulators (CD59 and DAF/CD55): Decay-Accelerating Factor (DAF/CD55) accelerates the decay of C3 convertases. CD59 (Protectin) prevents C9 polymerization into MAC. Both proteins are anchored to cell membranes via glycosylphosphatidylinositol (GPI) anchors. Somatic mutations in the X-linked PIGA gene impair GPI anchor synthesis, causing Paroxysmal Nocturnal Hemoglobinuria (PNH). Erythrocytes lacking CD55/CD59 suffer complement-mediated intravascular hemolysis, presenting with dark morning urine, venous thrombosis, and pancytopenia.