Mechanisms of Glomerular Injury Following Immune Complex Formation

Slide 1 · Mechanisms of Glomerular Injury

Following Immune Complex Formation

From immune-reactant localization to inflammatory injury and morphology

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Mechanism determines the lesion

Glomerular antigen–antibody complexes can provoke local inflammation.

The site of deposition is a major determinant of cellular access and injury.

Complement, Fc receptor engagement, and resident glomerular cells translate deposition into damage.

Formation or deposition of antigen–antibody complexes

Localization in a glomerular compartment

Effector-cell and soluble-mediator responses

Distinctive glomerular injury pattern

Slide 3 · Four questions organize immune-complex injury

Where do antigen–antibody complexes originate?

Where do they localize within the glomerulus?

Which cells and soluble mediators respond?

Does antigen exposure resolve or persist?

Origin: Local formation or circulation

Site: Mesangial, subendothelial, subepithelial

Effector system: Complement, cells, coagulation

Duration: Resolution or remodeling

Slide 4 · The filtration barrier defines exposure

Capillary blood contacts fenestrated endothelium first.

The glomerular basement membrane (GBM) separates endothelial from podocyte compartments.

Podocyte foot processes face Bowman urinary space on the outer GBM surface.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

SEPARATE INSET: Intercapillary mesangium

Discrete immune deposits (schematic)

Slide 5 · Mesangium is not behind the GBM

The mesangial compartment lies between capillary loops.

Unlike subepithelial sites, it is not shielded from the vascular lumen by the GBM.

This proximity helps explain mesangial inflammatory responses to immune deposits.

INTERCAPILLARY MESANGIUM

Capillary

Lumen

Capillary

Lumen

Mesangial

Matrix / cells

Mesangial deposits lie between adjacent capillary loops

Glomerular tuft schematic — not microscopy

Slide 6 · Antibodies initiate two major routes of injury

Antibodies can bind antigen in situ within glomeruli.

Circulating preformed antigen–antibody complexes may become trapped in the glomerulus.

Both routes can produce discrete immune deposits with granular immunofluorescence.

Local complex formation around glomerular antigens

OR deposition of circulating complexes

Granular deposits and downstream effector activation

Slide 7 · In situ complexes: Intrinsic tissue antigens

Antibodies recognize antigen molecules that are natural components of the glomerulus.

Antigen location controls where individual immune complexes assemble.

Membranous nephropathy exemplifies locally formed subepithelial complexes.

Intrinsic glomerular antigen

Specific circulating antibody binds in situ

Discrete complexes along outer capillary wall

Subepithelial electron-dense deposits

Slide 8 · Planted antigens enable local assembly

Non-glomerular antigens can lodge or bind within glomerular structures.

Examples include nuclear proteins, bacterial products, and aggregated immunoglobulins.

Antibody subsequently reacts at the planted antigen site.

Non-glomerular antigen localizes in glomerulus

Antigen remains accessible to antibody

Local antigen–antibody assembly

Immune deposit and possible inflammation

Slide 9 · Cationic planted molecules exploit charge

Cationic molecules may adhere to anionic glomerular components.

Electrostatic interactions help determine where antigen becomes planted.

The subsequent immune reaction occurs at that localization site.

Positively charged antigen

Binding to anionic glomerular components

Antibody access and local complex formation

Slide 10 · Nuclear antigens can be planted

DNA, nucleosomes, and other nuclear proteins may bind GBM components.

Such antigen localization permits antibody binding within the glomerulus.

Presence of a circulating autoantibody does not, by itself, reveal the deposition route.

Nuclear antigen with GBM affinity

Antigen becomes localized

Antibody reacts at tissue site

Immune deposit develops

Slide 11 · Circulating complexes do not require GBM specificity

Antigen and antibody react outside the glomerulus.

The antibody need not recognize an intrinsic renal structure.

Complexes localize through physicochemical properties and glomerular hemodynamics.

Antigen–antibody complex formed in circulation

Transport through renal microcirculation

Entrapment in glomerular compartment

Effector activation after deposition

Slide 12 · Endogenous sources of immune complexes

Endogenous antigens are implicated in systemic lupus erythematosus (SLE).

IgA nephropathy is included among examples of endogenous antigen-associated disease.

Tumor antigens are also proposed contributors to some immune-complex nephritides.

Endogenous antigen

Antibody binding

Circulating immune complexes

Variable glomerular deposition

Slide 13 · Exogenous antigen sources

Microbial antigen examples include streptococcal products.

Hepatitis B surface antigen and hepatitis C viral antigens may be implicated.

The textbook also lists Treponema pallidum and Plasmodium falciparum antigens.

Infectious antigen in host

Humoral response

Immune-complex generation

Renal deposition and injury

Slide 14 · Many triggering antigens remain unknown

An immune-complex pattern can be established without identifying the inciting antigen.

Do not infer a specific infection or autoimmune cause from granular staining alone.

Etiologic interpretation requires clinical and laboratory context beyond the deposit pattern.

Granular immune-reactant deposition

Mechanism recognized

Inciting antigen may still be uncharacterized

Slide 15 · Size and charge affect localization

Charge and size of immune reactants influence glomerular localization.

Large aggregated proteins may accumulate within the mesangium.

The physicochemical properties of complexes contribute to lesion heterogeneity.

Charge of antigen or immune complex

Size and aggregation

Preferential localization

Site-specific injury response

Slide 16 · Glomerular hemodynamics alter deposition

Flow through the glomerular microcirculation affects immune-complex trapping.

Hemodynamic conditions act alongside antigen properties.

A single antigen source need not result in a single structural lesion.

Glomerular capillary flow

Interaction with circulating complexes

Site-specific accumulation

Variation in pathologic pattern

Slide 17 · Mesangial handling affects immune deposition

Mesangial function is one determinant of immune-reactant localization.

Mesangial cells can participate in clearance of deposited material.

They can also be activated to release inflammatory and growth mediators.

Mesangial localization

Mesangial uptake and degradation

Activation or resolution

Potential proliferation and remodeling

Slide 18 · Barrier integrity modifies localization

The charge-selective filtration barrier helps influence deposition sites.

A change in capillary wall integrity may alter access to glomerular compartments.

Localization and injury should be interpreted as interacting processes.

Integrity of capillary-wall barrier

Accessibility of glomerular sites

Immune-reactant localization

Altered pattern of injury

Slide 19 · Circulating and in situ routes may coexist

A trapped circulating complex may retain free binding sites.

Additional antigen, antibody, or complement can interact locally.

Therefore, routine morphology may not prove one exclusive route of formation.

Circulating complex is trapped

Additional local binding interactions

Further immune-complex assembly

Combined or ambiguous mechanism

Slide 20 · Granular IF reflects discrete complexes

Immune-reactant deposits form discontinuous foci.

Immunofluorescence shows a granular pattern along capillary walls or in mesangium.

Granularity is characteristic of immune-complex localization, not one specific cause.

IMMUNOFLUORESCENCE PATTERN

Interrupted, coarse granular signal

Stylized IF pattern — not microscopy

Slide 21 · Linear IF points to a different mechanism

Anti-GBM antibodies recognize antigens distributed along the length of the GBM.

Bound antibody produces a diffuse linear immunofluorescence pattern.

Linear anti-GBM binding must be distinguished from granular immune-complex deposition.

IMMUNOFLUORESCENCE PATTERN

Continuous linear capillary-wall signal

Stylized IF pattern — not microscopy

Slide 22 · Immunofluorescence and electron microscopy differ

Immunofluorescence

Pattern: Granular or linear

Distribution: Capillary wall, mesangium, or both

Electron microscopy

Dense deposits: Position relative to GBM

Subendothelial, subepithelial, mesangial, or multiple

Slide 23 · Three main immune-deposit compartments

Subendothelial: Between endothelial cells and GBM.

Subepithelial: Between outer GBM surface and podocytes.

Mesangial: Within the mesangial compartment between capillary loops.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

SEPARATE INSET: Intercapillary mesangium

Discrete immune deposits (schematic)

Slide 24 · Subendothelial deposits remain blood-facing

Deposits lie beneath endothelium, on the luminal side of the GBM.

They readily provoke leukocyte infiltration and may be associated with crescents.

Subendothelial immune complexes are linked to membranoproliferative patterns.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

Discrete immune deposits (schematic)

Slide 25 · Subepithelial deposits are relatively shielded

Deposits sit outside the GBM beneath podocytes.

The GBM separates them from circulating leukocytes.

They generally do not trigger the same inflammatory leukocyte response as subendothelial deposits.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

Discrete immune deposits (schematic)

Slide 26 · Mesangial deposits have inflammatory access

Mesangial deposits are not separated from the capillary lumen by the GBM.

Leukocytic infiltration may occur.

Mesangial and endothelial cells may proliferate in the resulting lesion.

INTERCAPILLARY MESANGIUM

Capillary

Lumen

Capillary

Lumen

Mesangial

Matrix / cells

Mesangial deposits lie between adjacent capillary loops

Glomerular tuft schematic — not microscopy

Slide 27 · Localization predicts the response

Location

Relationship to blood

Typical response

Subendothelial

Luminal side of GBM

Leukocyte influx; Potentially crescents

Mesangial

Mesangium accessible from capillary lumen

Leukocytes; Mesangial/endothelial proliferation

Subepithelial

Separated from capillary lumen by GBM

Less leukocyte-rich inflammation; Podocyte injury

Slide 28 · Postinfectious humps are subepithelial

Postinfectious glomerulonephritis may show isolated dome-like subepithelial deposits.

The term “hump” describes deposit configuration on electron microscopy.

The isolated hump pattern differs from regularly distributed membranous deposits.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

Discrete immune deposits (schematic)

Slide 29 · Membranous pattern: Repeated subepithelial deposits

Membranous nephropathy classically has multiple discrete subepithelial deposits.

Host responses around deposits produce an apparent thickened GBM by light microscopy.

This pattern can occur without a leukocyte-rich capillary-lumen inflammatory response.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

Discrete immune deposits (schematic)

Slide 30 · Membranoproliferative pattern: Subendothelial injury

Subendothelial immune complexes may cause recurrent endothelial injury.

Repeated injury and repair can produce basement membrane double contours.

Membranoproliferative morphology is a reaction pattern rather than proof of one antigen.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

Discrete immune deposits (schematic)

Slide 31 · IgA nephropathy: Mesangial emphasis

Mesangial immune deposits as typical of IgA nephropathy.

Mesangial localization explains potential mesangial proliferation and inflammation.

Localization describes the injury site but does not alone establish the entire etiology.

INTERCAPILLARY MESANGIUM

Capillary

Lumen

Capillary

Lumen

Mesangial

Matrix / cells

Mesangial deposits lie between adjacent capillary loops

Glomerular tuft schematic — not microscopy

Slide 32 · Lupus may involve several compartments

Lupus-associated immune-complex injury may show subendothelial deposits.

Mesangial deposits may occur, including in early lupus nephritis.

Multiple sites can be involved within the same glomerular disease.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

SEPARATE INSET: Intercapillary mesangium

Discrete immune deposits (schematic)

Slide 33 · The same glomerulus may harbor multiple deposits

Electron-dense deposits may occur at more than one ultrastructural location.

No single simple compartment diagram fully describes every biopsy.

Pathologic interpretation must account for the complete distribution.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

SEPARATE INSET: Intercapillary mesangium

Discrete immune deposits (schematic)

Slide 34 · Immune deposits trigger inflammatory amplification

Deposited antibody can activate complement.

Antibodies may engage Fc receptors on circulating leukocytes.

These two routes recruit and activate effectors that injure glomerular tissue.

Deposited antigen–antibody complexes

Complement activation and Fc receptor engagement

Cell recruitment and mediator release

Glomerular inflammation and dysfunction

Slide 35 · C5a links complement to leukocyte influx

Complement activation generates chemoattractants, especially C5a.

Neutrophils and monocytes enter susceptible glomeruli.

The recruited cells can then damage GBM and resident cells.

Complement activation

C5a chemotaxis

Neutrophil/monocyte recruitment

Local protease and oxidant release

Slide 36 · Fc receptors contribute independent activation

Leukocyte Fc receptors recognize antibody-containing deposits.

Fc receptor engagement promotes leukocyte activation.

Inflammation may therefore reflect antibody–cell interactions as well as complement.

Immune complex exposes antibody Fc portions

Leukocyte Fc-receptor engagement

Leukocyte activation

Inflammatory mediator release

Slide 37 · Neutrophil proteases damage the GBM

Recruited neutrophils release proteolytic enzymes.

Protease activity can degrade GBM matrix.

Loss of capillary-wall integrity facilitates severe glomerular injury.

Neutrophil enters involved capillary

Protease release

GBM degradation

Capillary-wall injury

Slide 38 · Reactive oxygen species injure glomerular cells

Activated neutrophils release oxygen-derived free radicals.

Reactive oxidants damage glomerular cells.

Resident mesangial cells can also generate reactive oxygen species.

Leukocyte or mesangial activation

Reactive oxygen species

Cellular injury and local inflammatory amplification

Slide 39 · Arachidonic acid metabolites alter filtration

Neutrophil-derived arachidonic acid metabolites influence vascular tone.

These vascular changes contribute to reduced glomerular filtration rate (GFR).

Functional impairment may reflect mediators in addition to structural damage.

Inflammatory-cell activation

Arachidonic acid metabolite production

Glomerular vascular effects

Reduced GFR

Slide 40 · Subendothelial inflammation can produce crescents

Subendothelial deposits commonly elicit leukocyte-rich injury.

Severe injury may disrupt the GBM and leak plasma proteins into Bowman space.

Fibrin and coagulation mediators contribute to crescent formation.

Subendothelial immune-complex inflammation

Severe capillary-wall / GBM injury

Leakage and fibrin in Bowman space

Crescentic response

Slide 41 · Macrophages and T cells sustain inflammation

Infiltrating macrophages and activated T lymphocytes release multiple mediators.

These cells may contribute to injury initiated by immune complexes.

Experimental evidence for T-cell amplification is strongest in selected crescentic models.

Immune-mediated initiation

Macrophage and T-cell recruitment

Mediator release

Propagation of glomerular inflammation

Slide 42 · Activated mesangial cells are effector cells

Mesangial cells may produce cytokines, chemokines, oxidants, and growth factors.

They also release eicosanoids, nitric oxide, and endothelin.

Intrinsic-cell responses can occur without major leukocyte infiltration.

Immune-reactant deposition / glomerular stimulus

Mesangial activation

Inflammatory and vasoactive mediator release

Cellular and hemodynamic effects

Slide 43 · Platelets can amplify glomerular injury

Platelets may aggregate within inflamed glomeruli.

They release eicosanoids, growth factors, and other mediators.

These products may support vascular injury and glomerular cell proliferation.

Immune-mediated vascular injury

Platelet aggregation

Mediator and growth-factor release

Proliferation and vascular effects

Slide 44 · Complement has two major effector modes

C5a-driven pathway

Chemotaxis

Neutrophils and monocytes

Leukocyte-dependent injury

C5b–C9 pathway

Membrane attack complex

Cell injury or activation

Can operate without neutrophils

Slide 45 · C5b–C9 can injure cells without neutrophils

The C5b–C9 membrane attack complex can cause cell lysis.

It may stimulate mesangial cells to release oxidants and proteases.

Experimental membranous glomerulopathy demonstrates MAC-associated proteinuria without neutrophils.

Complement terminal pathway

C5b–C9 formation

Glomerular cell injury / stimulation

Protein leakage and other effects

Slide 46 · Cytokines support leukocyte adhesion

Interleukin-1 and tumor necrosis factor are identified glomerular injury mediators.

Both infiltrating leukocytes and resident cells can produce cytokines.

They promote adhesion and other inflammatory responses.

Activated glomerular / infiltrating cells

IL-1 and TNF release

Leukocyte adhesion and activation

Inflammatory amplification

Slide 47 · CCL2 recruits mononuclear cells

Monocyte chemoattractant protein-1 is also termed CCL2.

This chemokine promotes entry of monocytes and lymphocytes.

Cell recruitment can sustain glomerular inflammatory activity.

Local chemokine release

CCL2-mediated recruitment

Monocyte and lymphocyte influx

Local inflammation

Slide 48 · PDGF promotes mesangial proliferation

Platelet-derived growth factor (PDGF) contributes to mesangial cell proliferation.

Growth responses may accompany mesangial immune-complex inflammation.

Cellularity is therefore both an inflammatory and proliferative manifestation.

Immune-mediated / platelet-cell activation

PDGF release

Mesangial cell proliferation

Increased glomerular cellularity

Slide 49 · Fibrogenic growth factors drive remodeling

Transforming growth factor-β, connective tissue growth factor, and fibroblast growth factor favor matrix accumulation.

Extracellular matrix deposition and hyalinization contribute to sclerosis.

This links persistent immune injury to structural chronic damage.

Repeated or persistent injury

Fibrogenic growth-factor signaling

Extracellular matrix deposition

Glomerulosclerosis

Slide 50 · Hemodynamic mediators change renal function

Eicosanoids, nitric oxide, angiotensin, and endothelin influence glomerular hemodynamics.

Vascular responses can change filtration before extensive sclerosis appears.

Resident cells and infiltrating leukocytes contribute to mediator production.

Inflammatory mediator production

Change in glomerular vascular behavior

Alteration in filtration dynamics

Functional impairment

Slide 51 · VEGF helps maintain endothelial integrity

Vascular endothelial growth factor supports glomerular endothelial integrity.

It may participate in regulation of capillary permeability.

Barrier function reflects interactions among intrinsic glomerular cells.

Glomerular endothelial maintenance

VEGF-associated support

Integrity and permeability of capillary barrier

Slide 52 · Coagulation links vascular injury to crescents

Fibrin can accumulate within glomeruli and Bowman space.

This indicates local coagulation cascade activation.

Activated coagulation factors, particularly thrombin, may stimulate crescent formation.

Capillary-wall inflammatory injury

Coagulation cascade activation

Fibrin and thrombin

Crescent-forming response

Slide 53 · Podocyte injury is a distinct consequence

Immune mechanisms can injure podocytes, including antibodies to podocyte antigens.

Foot-process effacement and slit diaphragm disruption compromise filtration selectivity.

Proteinuria results when the barrier no longer retains plasma proteins.

Immune-mediated podocyte injury

Foot-process / slit-diaphragm alteration

Loss of protein selectivity

Proteinuria

Slide 54 · Subepithelial deposits can cause proteinuria

GBM shielding limits leukocyte access to subepithelial deposits.

Nevertheless, local complement-mediated epithelial injury may impair the filtration barrier.

Lack of a leukocyte-rich lesion does not imply absence of clinically relevant damage.

Subepithelial immune deposits

Limited circulating leukocyte contact

Podocyte / complement-mediated injury

Proteinuria

Slide 55 · Two pathways produce protein leakage

Inflammatory capillary-wall injury

Subendothelial / mesangial access

Leukocytes and capillary damage

Barrier disruption

Podocyte-dominant injury

Subepithelial or podocyte-directed events

Foot process / slit diaphragm damage

Albumin leakage

Slide 56 · Deposits can eventually be degraded

Neutrophils and macrophages can remove deposited immune complexes.

Mesangial cells also participate in degradation and clearance.

Endogenous proteases contribute to removal of deposited material.

Immune deposits in glomerulus

Cellular uptake and proteolysis

Reduced deposited material

Inflammation may subside

Slide 57 · Limited antigen exposure permits recovery

When antigen exposure is short-lived or limited, deposit clearance can outpace new deposition.

The inflammatory reaction may subside completely.

Most poststreptococcal glomerulonephritis cases illustrate this potentially reversible course.

Transient antigen exposure

Limited immune-complex deposition

Degradation of deposited complexes

Inflammation resolves; Recovery possible

Slide 58 · Persistent exposure drives recurrent remodeling

Prolonged antigen exposure can generate repeated immune-complex deposition.

Each episode can cause cellular injury and subsequent tissue remodeling.

SLE and viral hepatitis are textbook examples of prolonged or recurrent exposure.

Persistent or recurrent antigen

Repeated immune-complex deposition

Episodic inflammation and cell injury

Chronic remodeling of glomerular structures

Slide 59 · Location influences chronic morphologic pattern

Dominant deposit

Potential chronic pattern

Mechanistic rationale

Subepithelial

Membranous

Persistent outer capillary-wall deposition and GBM response

Subendothelial

Membranoproliferative

Recurring endothelial injury; Repair and matrix remodeling

Mesangial

Mesangial proliferative features

Mesangial exposure and growth response

Slide 60 · One viral antigen can produce different patterns

Hepatitis B or C antigens are associated with more than one immune-complex injury pattern.

Subepithelial in situ localization may yield membranous morphology.

Subendothelial antigen deposition or circulating complexes may yield membranoproliferative morphology.

One etiologic agent: Hepatitis B or C

Different sites / routes of immune-reactant deposition

Membranous OR membranoproliferative pattern

Slide 61 · C3 glomerulopathy is a related but distinct pathway

Alternative complement dysregulation characterizes C3 glomerulopathy.

Dense deposit disease and C3 glomerulonephritis are included under this category.

Immune complexes or monoclonal immunoglobulins may trigger or exacerbate dysregulation.

Defective alternative-complement regulation

Complement activation

C3 glomerulopathy phenotypes

Possible triggering role of immune complexes

Slide 62 · The role of T cells requires careful qualification

Activated T cells and macrophages occur in some human and experimental glomerulonephritides.

Experimental depletion and transfer studies support cellular contributions to injury.

The source explicitly notes no proof that human glomerulonephritis arises primarily from T-cell activation.

Antibody-mediated initiation in many diseases

T-cell / macrophage inflammatory contribution

Disease propagation in selected experimental systems

Primary T-cell etiology in humans unproven

Slide 63 · Inflammatory, membranous, and mesangial responses differ

Predominant setting

Cellular response

Pathologic emphasis

Subendothelial

Leukocyte-rich; Possible crescents

Proliferative and severe inflammatory lesions

Mesangial

Mesangial/endothelial proliferation

Mesangial inflammation and cellularity

Subepithelial

Typically less leukocyte-rich

GBM remodeling and podocyte dysfunction

Slide 64 · Clinically relevant functional consequences

Damage to the capillary wall can permit hematuria and impair GFR.

Podocyte and slit-diaphragm injury allow albumin leakage and proteinuria.

The combination and severity of lesions determine the clinical presentation.

Endothelial / GBM injury → hematuria and GFR decline

Podocyte filtration-barrier injury → proteinuria

Histologic lesion + functional phenotype

Slide 65 · A biopsy answers different mechanistic questions

Technique

What is evaluated

Interpretive caution

Light microscopy

Cellularity; Crescents; GBM thickening or sclerosis

Response pattern does not identify antigen alone

Immunofluorescence

Granular versus linear immune-reactant distribution

Does not prove in situ vs circulating route

Electron microscopy

Ultrastructural location of dense deposits

More than one compartment may be involved

Slide 66 · Interpret granular IF with EM localization

Start by identifying whether deposits are granular along loops, mesangium, or both.

Use electron microscopy to classify the deposit compartment.

Then connect the compartment to a plausible inflammatory or podocyte-dominant response.

IF: Granular capillary-wall / mesangial pattern

EM: Subendo / mesangial / subepi deposits

LM: Proliferation / GBM response / crescents

Clinicopathologic integration

Slide 67 · Diagnostic pitfall: Granularity is not etiology

Granular IF establishes a distribution pattern compatible with immune-complex deposition.

Multiple diseases share similar granular patterns.

Neither morphology nor pattern alone identifies the precise antigen or its source.

Observed granular staining

Support for immune-complex localization

Multiple plausible underlying causes

Clinical and laboratory correlation needed

Slide 68 · Diagnostic pitfall: IF cannot time deposition

Deposits may persist after the initiating antigen exposure has changed.

Local trapping can be followed by additional in situ immune-complex assembly.

A static biopsy cannot reliably reconstruct every prior event in complex formation.

Time-varying immune-complex generation

Deposition and local additions

Clearance or repeated deposition

Single biopsy = current morphology

Slide 69 · Clinicopathologic exercise: Subepithelial deposits

Hypothetical biopsy: Granular capillary-wall IF with numerous subepithelial dense deposits.

The absence of florid leukocytic exudation is mechanistically plausible.

The key injury may involve podocytes and progressive GBM remodeling.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

Discrete immune deposits (schematic)

Slide 70 · Clinicopathologic exercise: Subendothelial deposits

Hypothetical biopsy: Subendothelial dense deposits and prominent endocapillary inflammation.

Luminal proximity explains strong recruitment of leukocytes.

Severe capillary injury may be associated with crescent formation.

CAPILLARY LUMEN

Fenestrated endothelium

Glomerular basement membrane (GBM)

Podocyte foot processes

URINARY SPACE

Discrete immune deposits (schematic)

Slide 71 · Clinicopathologic exercise: Mesangial deposits

Hypothetical biopsy: Dominant mesangial immune deposits with mesangial hypercellularity.

Mesangial accessibility to capillary blood permits inflammatory-cell responses.

Intrinsic mesangial mediator production can add proliferative effects.

INTERCAPILLARY MESANGIUM

Capillary

Lumen

Capillary

Lumen

Mesangial

Matrix / cells

Mesangial deposits lie between adjacent capillary loops

Glomerular tuft schematic — not microscopy

Slide 72 · Mechanistic synthesis: Deposition to outcome

Antigen source and antibody

Formation / trapping of complexes

Site-specific effector activation

Morphology and renal dysfunction

Localization and persistence together shape the resulting glomerular lesion.

Slide 73 · Remember the four mechanistic discriminators

Question

High-yield answer

Where formed?

In situ or circulation; Both may contribute.

Where deposited?

Mesangial, subendothelial, subepithelial, or multiple sites.

How injured?

Complement, Fc receptors, recruited and resident cells, coagulation.

What determines outcome?

Exposure duration, deposit clearance, repeated injury, remodeling.

Slide 74 · Primary reference and interpretive boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026), Chapter 20: The Kidney.

Core section: “Mechanisms of Glomerular Injury Following Immune Complex Formation,” printed pp. 820–821.

Adjacent glomerular-injury, mediator, and podocyte sections (pp. 817–822) supply essential mechanisms and context.

All structural images in this lecture are original schematics, not patient specimens or reproduced textbook figures.