Systemic Immune Complex Disease

Slide 1 · Systemic Immune Complex Disease

Type III hypersensitivity • serum sickness • tissue injury

Pathogenesis, morphologic correlates and clinical interpretation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Describe how circulating antigen–antibody complexes produce type III tissue injury.

Explain the three phases of systemic immune complex disease and organ predilection.

Relate complement consumption, vascular injury, and morphology to clinical findings.

Distinguish self-limited serum sickness from persistent immune complex disorders.

Formation

Deposition

Inflammation

Organ injury and clinical manifestations

Slide 3 · Type III hypersensitivity is a deposition-driven lesion

Antigen–antibody complexes, usually containing IgG, initiate inflammation after depositing in tissue.

Complexes most often form in blood and subsequently localize to vessel walls.

The dominant injury is inflammatory—not simply the presence of antibody in plasma.

Circulating complexes

Tissue deposition

Inflammatory injury

Pathogenicity depends on location and host response

Slide 4 · The immune complex has two essential components

Antigen provides the molecular target for antibody binding.

Antibody binds antigen to create a multicomponent immune complex.

The resulting complexes can circulate and, under certain conditions, deposit in tissues.

Ag

Ag

Ag

Schematic antigen–IgG association (not to molecular scale)

Slide 5 · Why immune complex formation is not always disease

The relevant transition is from soluble complexes to tissue deposition with inflammation.

Whether formed complexes deposit is influenced by complex properties and local vascular factors.

These determinants are incompletely understood; Formation alone does not prove pathogenic injury.

Circulating complexes

No tissue injury established

Deposition + inflammation

Not every circulating complex causes disease

Slide 6 · Type III injury differs from direct antibody targeting

Type II pattern

Antibody binds a fixed target

Injury/dysfunction at the target

Type III pattern

Complexes form with soluble antigen

Deposits incite inflammation

Slide 7 · Exogenous antigens can trigger systemic disease

Foreign proteins introduced into the body can provoke an antibody response.

Microbial antigens also participate in selected immune complex disorders.

An external antigen is pathogenic in this model when it persists and encounters specific antibody.

Foreign or microbial antigen

IgG response

Antigen–IgG complexes

Exogenous antigen source

Slide 8 · Endogenous antigens can sustain complex formation

Autoantigens become complex-forming targets when self-reactive antibodies are produced.

Persistent autoantigen exposure supports continued formation of complexes.

Systemic lupus erythematosus is a paradigmatic chronic autoimmune setting.

Self antigen

Autoantibody

Persistent immune complexes

Autoimmunity can maintain antigen–antibody exposure

Slide 9 · Most complexes form in blood—not necessarily at the lesion

Circulating formation

Antigen + IgG in bloodstream

Complex later deposits in vessel/tissue

In situ formation

Antigen localized in tissue first

Antibody binds locally

Slide 10 · Systemic disease still has preferred target organs

Small blood vessels are common deposition sites: Vasculitis.

Glomeruli are vulnerable: Glomerulonephritis.

Joint structures are vulnerable: Arthritis or arthralgia.

Small vessels

Vasculitis

Kidney

Glomerulonephritis

Joints

Arthritis / arthralgia

Slide 11 · The three phases form one causal sequence

Antigen exposure and antibody response

Circulating complexes deposit

Complement / leukocyte inflammation

Vascular and organ injury

The same mechanism connects a delayed immune response to tissue-specific clinical findings.

Schematic causal sequence

Slide 12 · Serum sickness is the experimental and clinical prototype

Classic serum sickness followed exposure to large amounts of foreign serum proteins.

The model is especially useful because the order of antigen exposure, antibody formation, and injury is clear.

The model illustrates systemic type III hypersensitivity; It is not synonymous with every chronic immune complex disorder.

Foreign serum protein

Anti-protein IgG

Systemic inflammation

Prototype: Serum sickness

Slide 13 · Historical serum therapy revealed the syndrome

Foreign serum from immunized horses was once used to prevent infections such as diphtheria.

Recipients could subsequently develop serum sickness.

The history establishes the antigenic trigger; It does not imply that the original infection caused the immune complexes.

Exposure

Foreign serum

Immune response

IgG develops

Clinical reaction

Immune complexes

Slide 14 · Modern serum sickness is relatively infrequent

Modern serum sickness as uncommon.

The discussion emphasizes exposures to antibodies obtained from other individuals or species.

A history of relevant protein/antibody exposure provides etiologic context.

Administered foreign protein

Host antibody response

Immune complexes

Source-derived clinical context; No frequency estimate inferred

Slide 15 · Phase I: Antigen initiates a specific humoral response

The sequence begins when a protein antigen enters the host.

Antigen exposure elicits production of antibodies directed against that protein.

Complexes form only when sufficient antibody and persisting antigen overlap in time.

Protein antigen

Antibody response

Specific IgG

Phase I — formation

Slide 16 · The first antibody response requires an interval

In the prototype, antibodies typically appear about one week after protein injection.

This delay separates exposure from subsequent immune-complex formation.

The timing describes the model, not an invariant latency for every patient or re-exposure.

Day 0

Antigen given

~1 week

Antibody appears

After overlap

Complexes form

Slide 17 · Persistent antigen permits antigen–antibody overlap

Antigen may remain in circulation while the new antibody response develops.

Newly secreted antibody then reacts with residual circulating antigen.

The coexistence of antigen and antibody permits soluble complexes to assemble.

Ag

Ag

Ag

Antigen persistence permits binding by newly produced IgG

Slide 18 · Plasma antibodies encounter antigen in the circulation

Antibodies are secreted into blood and bind their cognate circulating antigen.

The resulting antigen–antibody complexes are not restricted to their site of entry.

Circulatory distribution makes injury possible in more than one organ.

Circulating antigen–antibody complexes

Slide 19 · Phase I can be summarized as an exposure–response overlap

Protein antigen introduced

Antibody response develops

IgG enters circulation

Residual antigen + IgG → complexes

A delayed antibody response explains why systemic signs follow, rather than coincide with, first antigen exposure.

Schematic causal sequence

Slide 20 · Phase II: Deposits form from circulating complexes

Complexes formed in the blood may lodge in vessel walls.

Deposition is a separate event from initial antigen–antibody binding.

Tissue-specific localization helps explain the organ pattern of disease.

Immune complex deposition at vessel wall

Slide 21 · Medium-sized immune complexes are especially implicated

Medium-sized complexes as relatively pathogenic.

The relationship is a source-described tendency rather than a numerical size cutoff.

Complex size contributes to deposition risk but does not independently predict disease.

Small

Not singled out

Medium

Relatively pathogenic

Large

Not singled out

Qualitative pattern; No numerical size boundaries given

Slide 22 · Slight antigen excess favors the pathogenic pattern

Medium-sized complexes formed with slight antigen excess are described as especially pathogenic.

The relative antigen–antibody balance influences complex characteristics.

Do not substitute an arbitrary antigen:antibody ratio; None is specified in the source.

Antigen

Antibody

Slight antigen excess

Context in which medium complexes are pathogenic

Slide 23 · Depositional behavior is incompletely explained

Why some immune complexes cause tissue disease is not fully understood.

Complex characteristics and vascular alterations as major influences.

Simple detection of immune complexes is therefore insufficient for an organ diagnosis.

Complex formation

Complex characteristics

Local vascular conditions

Both influence likelihood and sites of deposition

Slide 24 · Local vascular conditions modify where deposition occurs

The vessel environment contributes to trapping or retention of complexes.

Vascular factors act alongside the properties of the complexes.

This helps explain why the circulating trigger can yield focal or organ-predominant injury.

Immune complex deposition at vessel wall

Slide 25 · Filtration concentrates complexes at selected sites

High-pressure filtration can concentrate circulating complexes locally.

Kidney glomeruli filter blood to form urine.

Joint environments associated with synovial fluid production are another common target.

Circulating complexes

High-pressure filtration

Glomeruli • Joint environments

Slide 26 · Glomerular involvement follows renal filtration

Glomeruli are exposed to circulating complexes in a high-pressure filtering bed.

Deposition can provoke inflammation within glomerular structures.

The clinicopathologic result may be nephritis and abnormal protein leakage into urine.

Glomerular capillary tuft

Complex deposition → nephritis / proteinuria

Simplified conceptual schematic—not renal histology

Slide 27 · Synovial involvement explains joint symptoms

Complex deposition can occur at joints, another favored site.

Local inflammation produces joint pain and can manifest as arthritis.

Articular symptoms in systemic serum sickness belong to the same process as nephritis.

Joint / synovial region

Local complex-associated inflammation

Schematic only; Not anatomical scale

Slide 28 · Small vessels express the vascular lesion

Vessel walls commonly accumulate deposited complexes.

Immune-complex inflammation can injure the wall, producing vasculitis.

A vascular lesion provides a common mechanism linking several systemic manifestations.

Vessel-wall inflammation

Schematic vessel section; Not histology

Slide 29 · Organ predilection does not mean organ exclusivity

Kidney, joints, and small blood vessels as common sites—not the only sites.

The relative distribution of deposits determines the clinical pattern.

The same antigenic trigger can therefore be associated with multiple affected tissues.

Small vessels

Vasculitis

Kidney

Glomerulonephritis

Joints

Arthritis / arthralgia

Slide 30 · Phase III begins after complexes deposit in tissues

Deposited antigen–antibody complexes initiate acute inflammation.

Complement and leukocyte-mediated mechanisms turn deposits into damage.

Clinical illness follows the local tissue reactions rather than simple antigen exposure.

Tissue immune complexes

Inflammatory activation

Organ dysfunction

Phase III — inflammation and injury

Slide 31 · Complement is a central inflammatory amplifier

Complement components are detectable at sites of immune complex injury.

Activation supplies mediators that promote inflammatory cell recruitment and vascular change.

Consumption of complement during active disease helps explain reduced circulating C3.

Tissue immune complexes

Complement activation

Inflammatory fragments + C3 consumption

Slide 32 · C3a and C5a contribute to vascular inflammation

The related antibody-mediated inflammation section identifies C3a and C5a as anaphylatoxins.

These fragments increase vascular permeability; Complement products also recruit leukocytes.

Greater permeability and cell influx intensify inflammation at injured vessels.

Complement fragments

Vascular permeability

Leukocyte influx

Shared complement mechanism in antibody-mediated tissue injury

Slide 33 · Neutrophils and monocytes are effector cells

Complement-derived mediators recruit granulocytes and monocytes.

These cells accumulate near deposited immune complexes.

Cellular activation transforms the site into an acute inflammatory focus.

C3a / C5a and other mediators

PMN

PMN

Mo

Recruitment and local activation

Slide 34 · Fc and complement receptors activate recruited cells

Leukocytes respond to antibodies via Fc receptors.

Complement receptor engagement supplies an additional activation route.

Together these signals promote release of injurious inflammatory products.

IgG Fc portion

Complement products

Leukocyte activation

Both receptor routes can participate in injury

Slide 35 · Lysosomal enzymes directly damage tissue scaffolds

Activated leukocytes release lysosomal enzymes and proteases.

These enzymes can digest basement membrane, collagen, elastin, and cartilage.

Destruction of extracellular structures explains structural tissue injury at inflamed sites.

Activated leukocytes

Proteases / lysosomal enzymes

Basement membrane • collagen • elastin • cartilage

Slide 36 · Reactive oxygen species add tissue toxicity

Activated inflammatory leukocytes generate reactive oxygen species.

These mediators contribute to tissue injury after immune complex deposition.

The injury is therefore mediated by host effectors rather than physical bulk of the deposits alone.

Activated leukocytes

Reactive oxygen species

Tissue injury

A host-mediated effector mechanism

Slide 37 · Deposits drive a vascular inflammatory sequence

Complexes deposit in vessel wall

Complement / Fc receptor engagement

Neutrophil activation and mediators

Vasculitis ± necrosis

The deposition site is the site at which inflammatory injury becomes anatomically manifest.

Schematic causal sequence

Slide 38 · Acute vasculitis is the central tissue pattern

The morphology section identifies acute vasculitis as the principal immune complex lesion.

Vessel walls may show necrosis with intense neutrophilic inflammation.

The histologic pattern should be linked to the site of antibody/complement deposition.

Neutrophil-rich vascular inflammation

Conceptual vessel-wall schematic—not H&E evidence

Slide 39 · Fibrinoid necrosis reflects vessel-wall destruction

Necrotic tissue is admixed with immune complexes, complement, and plasma proteins.

The result is a smudgy eosinophilic appearance termed fibrinoid necrosis.

This term describes an observed morphologic pattern, not a unique antigen.

Fibrinoid necrosis

Conceptual vessel-wall schematic—not H&E evidence

Slide 40 · Immunofluorescence demonstrates granular deposits

In the kidney, immune complex deposits may appear as granular immunoglobulin and complement staining.

The granular pattern supports a deposited-complex mechanism.

A schematic cannot replace diagnostic immunofluorescence on an actual biopsy.

Granular Ig / complement deposits

Schematic only—not an IF micrograph

Slide 41 · Electron microscopy locates dense deposits

Immune complexes may be visible as electron-dense deposits along the glomerular basement membrane.

Ultrastructural demonstration provides anatomic evidence of deposited material.

The source supplies no specific distribution subtype to infer for all systemic immune complex diseases.

Glomerular basement membrane region

Electron-dense material

Conceptual depiction—not electron microscopy

Slide 42 · Morphologic diagnosis integrates several observations

Light microscopy

Vasculitis and wall necrosis

Inflammatory tissue reaction

IF / EM

Granular Ig + complement; Dense deposits

Depositional evidence

Slide 43 · The clinical syndrome reflects multisite inflammation

Systemic immune complex injury can cause fever, urticaria, joint pain, and lymph node enlargement.

Proteinuria signals possible glomerular involvement.

These manifestations arise during the inflammatory phase rather than as immediate direct toxicity of antigen.

Small vessels

Vasculitis

Kidney

Glomerulonephritis

Joints

Arthritis / arthralgia

Slide 44 · Fever and urticaria are part of acute serum sickness

Fever accompanies the systemic inflammatory phase.

Urticaria is among the cutaneous manifestations.

Their co-occurrence with joint or renal signs strengthens the multiorgan pattern.

Exposure

Antigen

~1 week

Complex formation

~10 days

Fever / urticaria

Slide 45 · Joint pain reflects inflammatory localization

Arthralgia occurs as immune complex–mediated inflammation affects joint tissues.

Immune complex deposition provides a mechanistic link to the systemic syndrome.

The prototype may involve joint pain together with vascular or glomerular abnormalities.

Joint / synovial region

Local complex-associated inflammation

Schematic only; Not anatomical scale

Slide 46 · Lymph node enlargement occurs with the systemic response

Lymphadenopathy is included among the acute inflammatory clinical features.

It should be interpreted in conjunction with exposure timing and other manifestations.

It is not a specific morphologic test for immune complex deposition.

Protein exposure

Humoral immune response

Systemic clinical findings

Lymphadenopathy is a syndrome component, not proof alone

Slide 47 · Proteinuria connects disease to glomerular injury

Glomerular immune complex deposition can elicit nephritic inflammation.

Proteinuria is specifically listed among clinical findings during the inflammatory phase.

The source does not define a diagnostic urine protein threshold for the syndrome.

Glomerular capillary tuft

Complex deposition → nephritis / proteinuria

Simplified conceptual schematic—not renal histology

Slide 48 · The prototype reaction becomes apparent after a delay

The inflammatory clinical phase at approximately ten days after antigen administration.

The interval follows antibody generation at about one week.

These are approximate observations from the prototype, not a rigid diagnostic clock.

0

Protein antigen

~7 days

Antibody response

~10 days

Inflammatory syndrome

Slide 49 · Serum C3 decreases when complement is consumed

Complement is engaged at deposition sites during active disease.

Active utilization can lower circulating serum C3.

Local complement detection and systemic complement consumption provide mutually consistent evidence.

Complement activation

Consumption of C3

Circulating C3 may fall

Slide 50 · C3 may track activity in selected patients

Serum C3 can sometimes be used to monitor disease activity.

A lower C3 is mechanistically interpretable as consumption during inflammation.

The text does not establish universal sensitivity, specificity, or numerical cutoffs.

Active complex injury

C3 consumption

Serum C3 may decrease

Useful in some cases; Interpret with the clinical and tissue findings

Slide 51 · Acute serum sickness may resolve after one exposure

A single large antigen exposure can produce an acute episode.

Lesions tend to resolve as immune complexes are catabolized.

The resolution model assumes that continued antigen exposure is absent.

Single exposure

Complex inflammation

Complex catabolism / resolution

Acute, potentially self-limited course

Slide 52 · Chronic immune complex disease requires persistence

Repeated or prolonged antigen exposure can maintain circulating complexes.

Continued antigen–antibody reactions can repeatedly drive deposition and injury.

The chronic model differs fundamentally from clearance after a single exposure.

Exposure

Antigen persists

Ongoing

Complex production

Repeated

Tissue injury

Slide 53 · Acute and chronic courses share the same mechanism

Acute serum sickness

Single major antigen exposure

Complexes are eventually catabolized

Lesions tend to resolve

Chronic complex disease

Repeated or sustained antigen

Continuing complexes and deposition

Inflammation may persist

Slide 54 · Systemic lupus erythematosus sustains autoantigen exposure

SLE features persistent antibody responses to self antigens.

Nuclear antigens as immune complex targets.

The resulting disease can affect kidneys, skin, joints, and other tissues.

Nuclear antigens

Autoantibody complexes

Nephritis / skin / joints

Example of endogenous antigen–driven disease

Slide 55 · Poststreptococcal nephritis illustrates microbial antigen

Streptococcal cell-wall antigen(s) in poststreptococcal glomerulonephritis.

Some antigen may be planted in the glomerular basement membrane.

Its principal manifestation in the source table is nephritis.

Glomerular capillary tuft

Complex deposition → nephritis / proteinuria

Simplified conceptual schematic—not renal histology

Slide 56 · Some polyarteritis nodosa has a viral antigen link

Hepatitis B virus antigens are listed for a subset of polyarteritis nodosa cases.

The clinical pathologic manifestation is systemic vasculitis.

Do not generalize the HBV association to all polyarteritis nodosa.

Vessel-wall inflammation

Schematic vessel section; Not histology

Slide 57 · Bacterial antigens appear in reactive arthritis

Bacterial antigens, exemplified by Yersinia, among immune complex disease associations.

The source table lists acute arthritis as the clinical manifestation.

This is a mechanism example table, not a claim that every reactive arthritis case is proven immune complex–mediated.

Joint / synovial region

Local complex-associated inflammation

Schematic only; Not anatomical scale

Slide 58 · The disease examples share mechanism, not identical phenotype

Example

Antigen source

Highlighted manifestation

Serum sickness

Foreign proteins

Arthritis, vasculitis, nephritis

SLE

Nuclear antigens

Nephritis, skin, arthritis

Poststreptococcal GN

Streptococcal antigens

Nephritis

Selected PAN

HBV antigens

Systemic vasculitis

Reactive arthritis

Bacterial antigens

Acute arthritis

Slide 59 · Systemic and local immune complex reactions differ by distribution

Systemic serum sickness

Circulating antigen and antibody

Deposits at multiple sites

Systemic clinical syndrome

Local Arthus reaction

Antigen introduced into sensitized site

Complexes form in a local vessel wall

Localized vasculitis / necrosis

Slide 60 · A vascular lesion alone does not establish Type III injury

Several antibody- and non–immune-complex mechanisms can inflame vessels.

A deposited-complex mechanism requires evidence interpreted with the clinical setting.

Look for antigenic context, complement findings, and relevant tissue deposition—not just vasculitis.

Vasculitis observed

Immune-complex evidence?

Other mechanisms possible

Morphology and mechanism are distinct diagnostic questions

Slide 61 · Clinical exercise: Delayed illness after a protein infusion

A patient develops fever, urticaria, arthralgia and proteinuria several days after a foreign protein exposure.

Question: Which sequence best links the exposure to these findings?

Reasoned answer: Antibody formation → circulating complexes → deposition → complement/leukocyte injury.

Delayed host IgG response

Systemic deposits

Inflammation / symptoms

Hypothetical teaching case; No patient data

Slide 62 · Clinical exercise: Low C3 with inflammatory nephritis

A patient has nephritis with granular immune deposits and reduced circulating C3.

Question: Which common mechanism connects these findings?

Reasoned answer: Local immune complex deposition with complement activation and consumption.

Granular deposits

Complement activation

C3 consumption

Hypothetical teaching case; No biopsy specimen depicted

Slide 63 · Four reasoning pitfalls to avoid

Antibody positivity does not, by itself, show tissue deposition.

A low C3 may support activity in some cases; Robbins gives no universal cutoff.

The prototype time intervals are approximate, not diagnostic criteria.

Vasculitis or nephritis does not, alone, identify the responsible antigen.

Exposure / antigen

Deposition evidence

Injury pattern

Require a coherent clinicopathologic mechanism

Slide 64 · Integrated mechanism: From blood to injured organ

Antigen persists while IgG emerges

Medium complexes circulate and deposit

Complement and leukocytes are activated

Vasculitis, nephritis, arthritis ± C3 reduction

A systemic trigger produces organ-predominant pathology through the distribution and inflammatory effects of deposits.

Schematic causal sequence

Slide 65 · Primary reference and scope

Robbins, Cotran & Kumar. Pathologic Basis of Disease, 11th edition (2026).

Chapter 6: Immune complex–mediated (Type III) hypersensitivity; systemic immune complex disease, pp. 187–188.

Adjacent source context: local immune complex disease, Table 6.4, Fig. 6.17, and morphology, pp. 188–189.

Conceptual figures in this deck are original editable schematics; no textbook photomicrographs are reproduced.