12.1 Principles of Immunology: Antigens, Antibodies, Complement, and Immune Responses

Principles of Immunology: Antigens, Antibodies, Complement, and Immune Responses

Clinical serology and immunology encompass the study of host defense mechanisms, the structural biology of humoral effectors, and the diagnostic application of antigen-antibody interactions. For the medical technologist preparing for the Philippine Medical Technologists Licensure Examination (MTLE) and international certifications (such as ASCPi), mastering the cellular dynamics of innate and adaptive immunity, the structural biochemistry of immunoglobulins, and the molecular cascades of the complement system provides the indispensable scientific foundation for interpreting diagnostic serological assays.

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1. Innate Versus Adaptive Immunity: Architecture of Host Defense

The human immune system maintains physiological homeostasis and defends against pathogenic invasion through two interrelated, coordinated arms: innate (natural, non-specific) immunity and adaptive (acquired, specific) immunity.

                                   ┌─────────────────────────────────────────┐
                                   │          Host Immune Defenses           │
                                   └────────────────────┬────────────────────┘
                                                        │
                    ┌───────────────────────────────────┴───────────────────────────────────┐
                    ▼                                                                       ▼
     ┌─────────────────────────────┐                                         ┌─────────────────────────────┐
     │   Innate (Non-Specific)     │                                         │     Adaptive (Specific)     │
     └──────────────┬──────────────┘                                         └──────────────┬──────────────┘
                    │                                                                       │
     ┌──────────────┴──────────────┐                                         ┌──────────────┴──────────────┐
     ▼                             ▼                                         ▼                             ▼
Anatomical/Chemical:         Cellular/Humoral:                         Cell-Mediated:                Humoral:
• Keratinized skin           • Neutrophils, Monocytes                  • T-Lymphocytes (CD4, CD8)    • B-Lymphocytes
• Mucociliary escalator      • Macrophages, Dendritic cells            • Perforin / Granzyme lysis   • Plasma cells
• Gastric acid (pH 1.5-2.0)  • NK cells (Missing self)                 • Cytokine orchestration      • Immunoglobulins
• Lysozyme (cleaves PG)      • Alternative/Lectin complement           • Memory T-cells              • Memory B-cells
• Acute phase reactants      • Interferons (IFN-α, IFN-β)              • Major histocompatibility    • Somatic hypermutation

Innate Immunity: Anatomical, Physiological, and Cellular Defenses

Innate immunity constitutes the phylogenetically older first line of defense. It operates continuously without prior sensitization, responds instantaneously (within minutes to hours), exhibits no immunological memory upon re-encounter, and targets invariant molecular patterns:

1. Physical and Chemical Barriers:

   • Stratified Squamous Epithelium (Skin): Keratinized mechanical barrier impermeable to intact microorganisms; sebaceous secretions supply lactic acid and long-chain unsaturated fatty acids (pH 5.5) inhibitory to bacterial colonization.
   • Mucous Membranes and Mucociliary Escalator: Epithelial goblet cells secrete viscous mucin that traps airborne particles; pseudostratified ciliated columnar respiratory epithelium propels mucus cephalad toward the pharynx.
   • Biochemical Secretions: Gastric hydrochloric acid (pH 1.5–2.0) sterilizes ingested contents; lysozyme (muramidase) in tears, saliva, and nasal secretions cleaves β(1→4) glycosidic linkages between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in bacterial peptidoglycan cell walls.

2. Cellular Effectors and Pattern Recognition:

   • Polymorphonuclear Neutrophils (PMNs): Short-lived circulating phagocytes (t1/2 ≈ 6–8 hours) that extravasate rapidly toward chemoattractants (C5a, leukotriene B4, IL-8). Neutrophils engulf pathogens into phagosomes, merging them with primary (azurophilic) granules containing myeloperoxidase (MPO), lysozyme, and defensins to generate lethal reactive oxygen species (ROS) via NADPH oxidase.
   • Monocytes and Macrophages: Long-lived tissue-resident phagocytes (e.g., Kupffer cells in liver, alveolar macrophages in lung, microglial cells in CNS, osteoclasts in bone) that orchestrate antigen clearance, wound debridement, and cytokine secretion (IL-1, TNF-α, IL-6).
   • Natural Killer (NK) Cells: Morphologically large granular lymphocytes expressing surface markers CD16 (low-affinity FcγRIII mediating Antibody-Dependent Cellular Cytotoxicity, ADCC) and CD56 without surface CD3 or TCR. NK cells evaluate target cells via the "missing-self" hypothesis: healthy nucleated cells express classical MHC Class I molecules that bind Killer Cell Inhibitory Receptors (KIRs), preventing lysis. Virus-infected or neoplastic cells that downregulate MHC Class I to escape CD8+ T cells fail to engage KIRs, allowing activating receptors (e.g., NKG2D) to trigger perforin- and granzyme-mediated target cell apoptosis.
   • Pattern Recognition Receptors (PRRs): Germline-encoded receptors—including Toll-Like Receptors (TLRs)—that recognize conserved Pathogen-Associated Molecular Patterns (PAMPs) on microbes: TLR4 recognizes Gram-negative lipopolysaccharide (LPS); TLR2 recognizes peptidoglycan and lipoteichoic acid; TLR3 binds double-stranded viral RNA; TLR5 binds bacterial flagellin; and TLR9 recognizes unmethylated bacterial CpG DNA motifs.

Adaptive Immunity: Cardinal Characteristics

Adaptive immunity develops following immunogen contact and is defined by five foundational hallmarks:

• Antigen Specificity: Distinguishes minute conformational differences between distinct antigenic determinants.
• Immunological Memory: Second and subsequent encounters with an antigen elicit an accelerated, higher-magnitude, and longer-lasting secondary (anamnestic) response dominated by high-affinity IgG.
• Vast Diversity: Somatic recombination of variable (V), diversity (D), and joining (J) gene segments catalyzed by RAG-1 and RAG-2 recombinases generates >10^11 distinct antigen receptor specificities.
• Self / Non-Self Discrimination: Central tolerance (clonal deletion of autoreactive thymocytes in the thymus and pre-B cells in bone marrow) and peripheral tolerance (anergy, regulatory T-cells) prevent catastrophic auto-reactivity.
• Clonal Selection: Antigens select and stimulate only those pre-existing lymphocyte clones bearing complementary surface receptors, driving clonal proliferation and differentiation into effector plasma cells and long-lived memory cells.

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2. Antigens, Immunogens, Epitopes, and Adjuvants

To understand serological diagnostics, the medical technologist must distinguish the physicochemical properties that govern immunogenicity versus antigenicity:

• Immunogen: Any biological substance capable of inducing a measurable immune response (humoral antibody production and/or cell-mediated immunity) upon introduction into a competent host.
• Antigen: Any substance capable of binding specifically to pre-formed antibodies or lymphocyte antigen receptors, regardless of whether it can stimulate their formation de novo.

         [ Immunogens ] ──► (Induce immune response + React with antibodies/TCRs)
               ▲
               │ (All immunogens are antigens, but not all antigens are immunogens)
               ▼
         [  Antigens  ] ──► (React with antibodies/TCRs; include Haptens)

Physicochemical Requirements for Immunogenicity

1. Foreignness: The molecule must be recognized as non-self; phylogenetic distance enhances immunogenicity.
2. Molecular Size: Highly potent immunogens typically have a molecular mass >100,000 Da (e.g., keyhole limpet hemocyanin, thyroglobulin). Substances with molecular mass <10,000 Da are generally weakly immunogenic, while substances <1,000 Da are non-immunogenic unless conjugated to a macromolecular carrier.
3. Chemical Composition and Complexity:
   • Proteins: The most potent immunogens due to structural diversity, three-dimensional folding, and diverse amino acid side chains.
   • Complex Polysaccharides: Good immunogens (e.g., bacterial capsular polysaccharides, ABO blood group antigens) due to repeating units, though they stimulate T-independent humoral responses.
   • Lipids and Nucleic Acids: Relatively poor immunogens in isolation; they become immunogenic when complexed with proteins (e.g., systemic lupus erythematosus autoantibodies against dsDNA complexed with histone proteins).
4. Degradability and Digestibility: Antigens must be enzymatically processed by antigen-presenting cells (APCs) to be presented on MHC molecules. Non-metabolizable synthetic polymers (e.g., D-amino acid polymers) cannot be cleared or presented, failing to provoke robust immunity.

Epitopes and Haptens

• Epitope (Antigenic Determinant): The discrete, localized spatial domain of an antigen that physically contacts the antigen-binding site (paratope) of an antibody. Epitopes may be linear (continuous)—determined by a contiguous sequence of amino acids (recognized even after denaturation)—or conformational (discontinuous)—formed by amino acid residues brought into juxtaposition by native tertiary protein folding (destroyed by heat denaturation).
• Hapten: A low-molecular-weight chemical substance (typically <1,000 Da, such as drugs, hormones, dinitrophenol, penicillin metabolites) that is antigenic but not immunogenic alone. A hapten can bind to pre-existing specific antibodies, but cannot induce de novo antibody synthesis unless covalently coupled to a high-molecular-weight carrier protein (e.g., bovine serum albumin). Once immunization occurs, antibodies are formed against the hapten, against the carrier, and against the junctional neoepitopes.
• Adjuvant: A substance that, when mixed with an immunogen and injected, non-specifically enhances the magnitude and duration of the immune response. Adjuvants do not form covalent chemical bonds with the immunogen. Mechanisms include creating an insoluble antigen depot for prolonged, slow release (e.g., aluminum hydroxide or aluminum phosphate [alum]), recruiting inflammatory dendritic cells, and activating pattern recognition receptors (e.g., Freund's Complete Adjuvant [FCA], which contains heat-killed Mycobacterium tuberculosis suspended in mineral oil).

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3. Immunoglobulin Architecture, Genetics, and Proteolytic Cleavage

Immunoglobulins (antibodies) are bifunctional heterodimeric glycoproteins synthesized by terminally differentiated plasma cells. They mediate antigen binding via their variable domains and trigger biological effector functions via their constant domains.

                                Antigen-Binding Paratope
                                     VL       VH
                                    [   ]   [   ]
                                    │   │   │   │
                                    [CL ]   [CH1]
                     Light Chain ──►│   │   │   │◄── Heavy Chain
                                    └───┘   └───┘
                         - - - - - - - - - - - - - - - - - - - - -  Papain cleavage (ABOVE hinge)
                                        |   |
                                      -S-S-S-S-     ◄── Hinge Region (Disulfide Bonds)
                         - - - - - - - - - - - - - - - - - - - - -  Pepsin cleavage (BELOW hinge)
                                        [CH2]   [CH2]
                                        │   │   │   │  ◄── Complement (C1q) binding site
                                        [CH3]   [CH3]
                                        │   │   │   │  ◄── Macrophage FcR / FcRn binding site
                                        └───┘   └───┘
                                              Fc

Basic Four-Chain Polypeptide Monomer

A monomeric immunoglobulin unit consists of four polypeptide chains: two identical heavy (H) chains (approx. 50–75 kDa each) and two identical light (L) chains (approx. 25 kDa each), yielding a total molecular mass of approximately 150 kDa for an IgG molecule. The chains are held together by covalent interchain disulfide bonds (-S-S-) and non-covalent hydrophobic and electrostatic forces.

1. Light Chains: Light chains occur in two distinct isotypes: kappa (κ) and lambda (λ). A single immunoglobulin molecule contains either two κ chains or two λ chains, never a hybrid mixture. In normal human serum, the overall κ:λ ratio is approximately 2:1. Deviation from this ratio (e.g., >3:1 or <1:1) indicates monoclonal B-cell proliferation (e.g., Multiple Myeloma). Each light chain possesses one variable domain (VL) and one constant domain (CL).
2. Heavy Chains: Heavy chains define the five primary immunoglobulin isotypes (classes):
   • gamma (γ) → IgG (subclasses γ1, γ2, γ3, γ4)
   • alpha (α) → IgA (subclasses α1, α2)
   • mu (μ) → IgM
   • delta (δ) → IgD
   • epsilon (ε) → IgE
3. Variable and Constant Domains:
   • Variable Domains (VH, VL): Located at the amino-terminal (N-terminal) ends. Within each variable domain lie three hypervariable loops termed Complementarity-Determining Regions (CDR1, CDR2, CDR3), framed by invariant framework regions (FRs). Together, the three CDRs of VH and three CDRs of VL fold into the three-dimensional cleft known as the paratope, which confers exquisite specificity for a single epitope.
   • Constant Domains (CH, CL): Located toward the carboxyl-terminal (C-terminal) ends. IgG, IgA, and IgD heavy chains possess three constant domains (CH1, CH2, CH3). IgM and IgE heavy chains lack a flexible hinge region and instead possess four constant domains (CH1, CH2, CH3, CH4).
4. Hinge Region: A proline-rich polypeptide segment situated between CH1 and CH2 domains in IgG, IgA, and IgD. Proline residues prevent α-helical coiling, conferring segmental flexibility that allows the two antigen-binding arms to move independently and span variable distances between epitopes on complex cellular antigens.

Proteolytic Enzymatic Cleavage (High-Yield MTLE Board Topic)

Enzymatic digestion experiments historically elucidated immunoglobulin domain structure:

• Papain Digestion:
  • Enzyme origin: Proteolytic cysteine protease derived from papaya latex.
  • Cleavage site: Cleaves heavy chains above the interchain disulfide bonds in the hinge region.
  • Digestion products: Cleaves one monomer into three separate fragments:
    1. Two identical Fab fragments (Fragment antigen-binding): Each Fab fragment contains an intact light chain (VL + CL) linked by a disulfide bond to the VH + CH1 segment of a heavy chain (molecular weight ≈ 45–50 kDa each). Each Fab is monovalent: it can bind a single epitope, but cannot cross-link antigen molecules to form visible precipitation or agglutination lattices.
    2. One crystallizable Fc fragment (Fragment crystallizable): Composed of the paired carboxy-terminal CH2 and CH3 domains held together by interchain disulfide bonds (molecular weight ≈ 50 kDa). The Fc fragment lacks antigen-binding capacity but mediates all biological effector functions: binding to classical complement component C1q, binding to cellular Fc receptors on macrophages, neutrophils, and NK cells, and binding to the neonatal Fc receptor (FcRn) for placental transfer.
• Pepsin Digestion:
  • Enzyme origin: Gastric aspartic endopeptidase.
  • Cleavage site: Cleaves heavy chains below the interchain disulfide bonds in the hinge region.
  • Digestion products: Generates one bivalent F(ab')2 fragment and degraded Fc subfragments:
    1. One F(ab')2 Fragment: Molecular weight ≈ 100 kDa. Retains both Fab-like arms joined together by the intact hinge disulfide bonds. Because it is bivalent (possessing two antigen-binding paratopes), an F(ab')2 fragment can bind, cross-link, and form visible precipitation and agglutination lattices.
    2. Degraded Fc Fragments: Pepsin extensively digests the Fc region into multiple small peptides (including a small pFc' fragment), destroying all Fc-mediated effector functions (complement fixation and Fc receptor binding).

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4. The Five Immunoglobulin Isotypes: Structural and Clinical Profiles

Clinical and Biochemical Specifics of Isotypes

1. Immunoglobulin G (IgG):
   • Predominance: Constitutes 75% to 80% of total circulating adult serum immunoglobulins.
   • Subclasses: Four subclasses: IgG1 (65%), IgG2 (23%), IgG3 (8%), and IgG4 (4%). Complement fixation potency follows: IgG3 > IgG1 > IgG2; IgG4 cannot fix complement via the classical pathway.
   • Placental Transfer: IgG is the only immunoglobulin class capable of crossing the human placenta. Transfer is mediated by the neonatal Fc receptor (FcRn) on syncytiotrophoblasts, providing passive immunity to the neonate that wanes over the first 4 to 6 months of life. Transplacental IgG also drives Hemolytic Disease of the Fetus and Newborn (HDFN / erythroblastosis fetalis) when maternal anti-Rh (anti-D) antibodies cross into fetal circulation.
   • Opsonization: IgG1 and IgG3 bind with high affinity to FcγRI (CD64) on macrophages and neutrophils, facilitating target phagocytosis.
2. Immunoglobulin M (IgM):
   • Structure: In serum, IgM exists as a heavy pentamer (900 kDa) comprising five monomeric subunits arranged in a planar ring connected by interchain disulfide bonds and a single J (joining) chain (15 kDa). While theoretically decavalent, steric hindrance generally limits functional antigen binding to 5 multivalent epitopes.
   • Earliest Antibody: IgM is the first isotype synthesized during embryonic and fetal development (ontogeny, beginning at ≈ 20 weeks gestation) and the first antibody produced following primary immunogen exposure. Elevated cord blood IgM indicates intrauterine infection (e.g., TORCH panel: Toxoplasma, Rubella, CMV, HSV).
   • Complement Fixation: Pentameric IgM is the most potent activator of the classical complement pathway. A single antigen-bound IgM pentamer in the "staple" conformation can bind C1q to initiate the cascade, whereas at least two adjacent IgG molecules are required.
   • Diagnostic Role: Intravascular localization makes IgM ideal for cold agglutination (e.g., cold agglutinin disease, Mycoplasma pneumoniae anti-I). Presence of specific IgM confirms acute or recent infection.
3. Immunoglobulin A (IgA):
   • Forms: Exists as a monomer in serum, but functions predominantly as a dimer in external mucosal secretions (colostrum, breast milk, saliva, tears, bronchial mucus, gastrointestinal fluids, and genitourinary secretions).
   • Secretory Component: Dimeric IgA is joined by a J chain. When dimeric IgA binds the polymeric immunoglobulin receptor (pIgR) on the basolateral surface of epithelial cells, it is endocytosed and transported across the cell. During release at the apical luminal surface, enzymatic cleavage leaves a 70-kDa fragment termed the Secretory Component (SC) bound to the dimer. The Secretory Component shields secretory IgA (sIgA) from proteolytic degradation by digestive enzymes (pepsin, trypsin) in mucosal secretions.
   • Pathology: Selective IgA Deficiency is the most common primary immunodeficiency (1:400 to 1:600 in Caucasians). Patients are at risk for recurrent sinopulmonary and gastrointestinal (Giardia lamblia) infections and may develop anti-IgA antibodies, leading to fatal anaphylactic transfusion reactions upon administration of blood components containing trace IgA.
4. Immunoglobulin D (IgD):
   • Expression: Present in trace amounts in serum (<1%) with a short half-life (2–3 days). Co-expressed with monomeric IgM on the surface of mature, naive B-lymphocytes via alternative RNA splicing of primary heavy chain transcripts.
   • Function: Serves as a B-cell antigen receptor signaling B-cell activation, differentiation, and survival; loss of surface IgD indicates B-cell memory or class switching.
5. Immunoglobulin E (IgE):
   • Concentration: Lowest circulating concentration of all immunoglobulins (<0.005% of total), with the shortest serum half-life (2–3 days). Highly thermolabile (56°C for 30 minutes irreversibly destroys the Fc domain).
   • Effector Mechanisms: The Fc domain of IgE binds with extreme affinity (Ka ≈ 10^10 M^-1) to FcεRI receptors on the surfaces of tissue mast cells and circulating basophils. When multivalent allergens cross-link adjacent membrane-bound IgE molecules, rapid intracellular signaling triggers exocytosis of preformed granules containing histamine, heparin, tryptase, and chemotactic factors, alongside de novo synthesis of leukotrienes (LTC4, LTD4, LTE4) and prostaglandins. This drives Type I immediate hypersensitivity reactions (anaphylaxis, asthma, urticaria).
   • Host Defense: Crucial for anti-parasitic immunity against invasive helminths (e.g., Schistosoma, Ascaris, Trichinella). IgE coats the parasite, recruiting eosinophils via their FcεRII (CD23) receptors to release Major Basic Protein (MBP) and eosinophil cationic protein, killing the helminth.

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5. The Complement System: Pathways, Activation, and Biological Split Products

The complement system is a tightly regulated network of over 30 soluble serum proteins, cell-surface receptors, and regulatory molecules synthesized primarily by hepatocytes. It operates as an enzymatic amplification cascade that lyses invading pathogens, opsonizes foreign particles, and recruits inflammatory cells.

   CLASSICAL PATHWAY               LECTIN PATHWAY              ALTERNATIVE PATHWAY
  Ag-Ab Complex (IgM, IgG)    MBL / Ficolin + Mannose        Spontaneous C3 Hydrolysis
             │                               │                       (C3 tickover) + LPS
             ▼                               ▼                                │
          C1q,r,s                        MASP-1, MASP-2                       ▼
             │                               │                            Factor B + D
             └───────────────┬───────────────┘                                │
                             ▼                                                ▼
                        Cleave C4 + C2                                   Cleave Factor B
                             │                                                │
                             ▼                                                ▼
                     C4b2a (C3 Convertase)                         C3bBb (C3 Convertase)
                             │                                                │
                             ├───────────────────────┬────────────────────────┤
                             ▼                       ▼                        ▼
                          Cleave C3               Cleave C3                Cleave C3
                       (C3a anaphylatoxin)     (C3b opsonin)            (Properdin stabilizes)
                             │                       │                        │
                             ▼                       ▼                        ▼
                    C4b2a3b (C5 Convertase) ◄──────────────────► C3bBbC3b (C5 Convertase)
                                                     │
                                                     ▼
                                                 Cleave C5 ──► C5a (Chemotaxin / Anaphylatoxin)
                                                     │
                                                     ▼ (C5b)
                                                Recruits C6, C7
                                                     │
                                                     ▼
                                                Recruits C8 + C9 (10-16 molecules)
                                                     │
                                                     ▼
                                          Membrane Attack Complex
                                               (MAC: C5b-9)
                                         10 nm Hydrophilic Pore ──► Cell Lysis

The Three Complement Activation Pathways

1. The Classical Pathway (Immune Complex-Initiated):

   • Trigger: Initiated by the binding of antigen-antibody immune complexes to C1q. Soluble pentameric IgM (a single molecule) or IgG (at least two molecules of IgG1, IgG2, or IgG3 in close proximity, ≤ 30 nm) binds the globular heads of C1q.
   • C1 Complex Assembly: C1 is a calcium-dependent macromolecular complex composed of one C1q molecule and two molecules each of the serine proteases C1r and C1s (C1qr2s2). Binding of C1q induces autoactivation of C1r, which in turn cleaves and activates C1s.
   • C3 Convertase Formation: Activated C1s cleaves native C4 into C4a (anaphylatoxin) and C4b. C4b binds covalently to the microbial surface. C1s then cleaves C2 into C2b and C2a. C2a binds to C4b to assemble C4b2a, the Classical Pathway C3 Convertase (dependent on Mg2+).
   • C5 Convertase Formation: C4b2a cleaves hundreds of C3 molecules into C3a and C3b. A molecule of C3b covalently associates with C4b2a to form C4b2a3b, the Classical Pathway C5 Convertase.

2. The Lectin Pathway (Mannose-Initiated):

   • Trigger: Antibody-independent. Initiated when circulating Mannose-Binding Lectin (MBL) or ficolins bind terminal mannose, fucose, or N-acetylglucosamine residues on bacterial, fungal, or viral cell walls.
   • Enzyme Activation: MBL structurally resembles C1q and circulates in complex with MBL-Associated Serine Proteases (MASP-1, MASP-2, MASP-3). MASP-2 acts analogously to C1s, directly cleaving C4 and C2 to assemble the identical C3 convertase (C4b2a) and C5 convertase (C4b2a3b).

3. The Alternative Pathway (Properdin / Tickover Pathway):

   • Trigger: Continuous, low-level spontaneous antibody-independent surveillance. Spontaneous hydrolysis of the internal thioester bond of native plasma C3 generates C3(H2O) (termed C3 tickover).
   • Enzyme Assembly: C3(H2O) binds Factor B in the presence of Mg2+. Factor D (a plasma serine protease) cleaves bound Factor B into Ba and Bb, forming the initial fluid-phase C3 convertase, C3(H2O)Bb. This convertase cleaves native C3 into C3a and C3b.
   • Surface Amplification: Generated C3b binds covalently via its exposed reactive thioester bond to nearby pathogen cell surface hydroxyl or amine groups (e.g., zymosan from yeast, bacterial LPS, teichoic acid). Surface-bound C3b binds additional Factor B, which is cleaved by Factor D to form C3bBb, the Alternative Pathway C3 Convertase.
   • Stabilization by Properdin: The C3bBb complex has a half-life of only ≈ 90 seconds. Properdin (Factor P) binds to C3bBb, stabilizing the complex and increasing its functional half-life nearly 10-fold to prevent rapid decay.
   • C5 Convertase Formation: As C3bBb cleaves more C3, an additional C3b binds to the convertase to form C3bBbC3b, the Alternative Pathway C5 Convertase.

The Common Terminal Lytic Pathway (Membrane Attack Complex, MAC)

All three pathways converge at the cleavage of native C5 by their respective C5 convertases (C4b2a3b or C3bBbC3b):

1. C5 Cleavage: The C5 convertase cleaves native C5 into C5a (released into fluid phase) and C5b (transiently bound to the membrane).
2. Sequential MAC Assembly: C5b sequentially binds one molecule of C6, then one molecule of C7. The resulting trimolecular C5b-6-7 complex inserts into the hydrophobic lipid bilayer of the target cell membrane.
3. C8 and C9 Polymerization: C5b-6-7 recruits one molecule of C8, which penetrates deeper into the membrane. This triggers the binding and rapid polymerization of 10 to 16 molecules of C9 into a rigid, hollow cylindrical tubule.
4. Osmotic Lysis: The fully assembled Membrane Attack Complex (MAC, C5b-9) forms a hydrophilic transmembrane pore approximately 10 nm in diameter. Water and sodium rush into the target cell down osmotic gradients while cytoplasmic macromolecules leak out, causing catastrophic cell swelling and rapid osmotic lysis. Terminal pathway deficiencies (C5, C6, C7, C8, C9) confer extreme clinical susceptibility to recurrent, invasive bacteremia caused by Neisseria species (Neisseria meningitidis and Neisseria gonorrhoeae).

Biological Functions of Complement Split Products

• Anaphylatoxins (C3a, C4a, C5a): Small peptide fragments cleaved during cascade activation. They bind specific G-protein-coupled receptors on mast cells and basophils, stimulating immediate degranulation and release of histamine and vasoactive amines. This causes smooth muscle contraction, arteriolar vasodilation, and increased post-capillary venule permeability. Potency hierarchy: C5a > C3a >> C4a.
• Chemotaxin (C5a): Potent chemoattractant for polymorphonuclear neutrophils, monocytes, and macrophages. C5a directs leukocyte migration along a concentration gradient toward sites of acute inflammation, stimulates leukocyte adhesion to vascular endothelium, and activates the phagocytic oxidative burst.
• Opsonins (C3b, iC3b): C3b and its inactive breakdown fragment iC3b deposit covalently in high density on microbial capsules and outer membranes. Professional phagocytes express Complement Receptor 1 (CR1 / CD35), which binds C3b, dramatically accelerating phagocytic engulfment and clearance.