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Immune Complex Disease: Deposition, Complement Activation, and Tissue Damage

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Classical, Alternative, and Lectin Complement Pathways Hypersensitivity Reactions: Types I–IV→→Proliferative Glomerulonephritis: Immune Deposition and Glomerular Cell Proliferation

Core Idea

Circulating antigen-antibody complexes deposit in tissues (kidney, skin, joints) and activate complement, recruiting leukocytes and causing inflammation. Serum sickness from foreign proteins (antibiotics, antivenom) or autoimmune conditions (lupus) exemplify immune complex disease; C3 deposition on glomeruli causes proliferative GN.

Explainer

You have already studied the four types of hypersensitivity reactions. Immune complex disease is the mechanism behind Type III hypersensitivity — and the key to understanding it is recognizing that the damage here is not caused by the antibody attacking host tissue directly (that is Type II), but by the antibody-antigen complex being deposited *in* tissue after forming in the bloodstream. It is collateral damage: the immune system cannot cleanly clear the complex, and in trying to do so, destroys the tissue it lands in.

Here is the sequence: antigen (a foreign protein, drug hapten, or self-antigen in autoimmunity) circulates in blood and binds circulating IgG or IgM antibodies. Normally, complement and phagocytes clear these immune complexes quickly. But when complexes are formed in large amounts, at a particular size, or in a host with impaired clearance, they persist in circulation and begin depositing in vessel walls and filtration organs. The glomeruli of the kidney, the synovium of joints, and small dermal capillaries are particularly vulnerable because blood is filtered or slowed through these structures under pressure, physically trapping the complexes.

Once deposited, immune complexes activate the complement cascade through the classical pathway — the same cascade you already know, now triggered not by a pathogen surface but by the Fc regions of antibodies in the complex. Complement activation generates C3a and C5a (anaphylatoxins), which recruit neutrophils and increase vascular permeability. Neutrophils attempt to phagocytize the complexes but cannot — the complexes are embedded in the basement membrane — so neutrophils release their granule contents (proteases, reactive oxygen species) extracellularly, damaging the surrounding tissue. The result is vasculitis, glomerulonephritis, and arthritis — the classic triad of serum sickness.

Serum sickness is the prototypical example: days after exposure to a foreign protein (antivenom, monoclonal antibodies, certain antibiotics), patients develop fever, rash, joint pain, and sometimes kidney involvement. The delay (7–14 days) reflects the time needed to mount an antibody response to the foreign antigen — only after antibodies are generated do complexes form and deposit. In contrast, lupus nephritis represents chronic immune complex disease: auto-antibodies against nuclear antigens (anti-dsDNA) form complexes that deposit in glomeruli over years, with C3 staining on biopsy as the pathological signature. Understanding that the same immune complex mechanism drives both acute serum sickness and chronic lupus nephritis shows how pathophysiology generalizes: the antigen changes, but the mechanism is identical.

Practice Questions 5 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Acid-Base Chemistry → Weak Acid Ionization → Weak Base Ionization → Acid and Base Strength: Ka, Kb, and Ionization → Leaving Groups and Nucleofugality → SN2 Substitution Reactions → SN1 Substitution Reactions → E1 Elimination Reactions → Alcohols and Ethers: Structure, Properties, and Nomenclature → Reactions of Alcohols → Aldehydes and Ketones: Structure and Reactivity → Oxidation Reactions in Organic Chemistry → Oxidation of Alcohols to Aldehydes and Ketones → Aldehyde and Ketone Structure and Nomenclature → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → IUPAC Nomenclature of Carbonyls and Carboxylic Acids → IUPAC Nomenclature of Alkenes → Electrophilic Addition to Alkenes → Aromaticity and Benzene → Electrophilic Aromatic Substitution (EAS) → Nucleophilic Aromatic Substitution (SNAr) → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Peptide Bonds and Polypeptide Formation → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Ion Channels and Selective Permeability Mechanisms → Osmotic Regulation and Cellular Water Balance → Osmosis and Tonicity → Active Transport → Cell Signaling and Signal Transduction → Homeostasis and Feedback Loops → Cardiovascular System Overview → Blood Composition and Function → Innate Immune Response → Inflammation and Wound Healing → Foundations of Immunology → Innate Immune System Components → Adaptive Immunity and Lymphocyte Diversity → Major Histocompatibility Complex Structure and Function → Antigen Processing and Presentation Pathways → Dendritic Cells and Professional Antigen-Presenting Cells → MHC Class II Antigen Presentation Pathway → CD4+ Helper T Cell Differentiation and Function → B Cell Activation and Germinal Center Responses → Class Switch Recombination and Isotype Switching → Antibody Isotypes and Effector Functions → Type II Hypersensitivity: Antibody-Mediated Cytotoxic Reactions → Type I Hypersensitivity: Allergic Reactions and IgE → Hypersensitivity Reactions: Types I–IV → Immune Complex Disease: Deposition, Complement Activation, and Tissue Damage

Longest path: 228 steps · 1222 total prerequisite topics

Prerequisites (2)

Hypersensitivity Reactions: Types I–IVhard Classical, Alternative, and Lectin Complement Pathwayshard

Leads To (1)

Proliferative Glomerulonephritis: Immune Deposition and Glomerular Cell Proliferationsoft