Opsonization and Phagocytosis

Slide 1 · Opsonization andPhagocytosis

Antibody-mediated (type II) hypersensitivity

Cell-surface recognition → effector recruitment → cellular injury

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Define opsonization and explain the role of IgG and C3b

Trace antibody-coated target recognition through phagocytic destruction

Differentiate phagocytosis, membrane attack, and ADCC

Apply mechanisms to immune blood-cell destruction

Slide 3 · Type II hypersensitivity: Antigen is target-bound

Type II responses depend on antigen location and the effector pathway

1

Cell-surface or matrix antigen

2

IgG antibody binds target

3

Antibody-dependent effector response

4

Cell depletion, inflammation, or dysfunction

Slide 4 · The antigen location determines the injury pattern

Cell destruction and tissue inflammation are different outcomes

Circulating cell

Antibody coats accessible cell surface

Phagocytic removal may deplete the population

Fixed tissue

Antibody deposits on matrix or basement membrane

Local inflammation may damage surrounding tissue

Slide 5 · Which antigens may become antibody targets?

Both self and acquired determinants may initiate antibody-mediated damage

Intrinsic antigens

Normal cell or tissue structures

Autoantibodies can recognize host targets

Acquired antigens

Exogenous drug or microbial antigens on a surface

Antibody can attack a cell bearing the acquired determinant

Slide 6 · Three mechanisms of antibody-mediated injury

Not every antibody-coated cell is destroyed by phagocytosis alone

1

Opsonization → phagocytosis

2

Complement terminal attack → lysis

3

Fc-mediated ADCC → target killing

Sequential causal pathway

Slide 7 · Opsonization: Coating makes a target recognizable

The opsonin creates a molecular bridge between target and phagocyte

Targetcell

Phagocyte

Fc recognition

Engulfment →phagolysosome

IgG-coated cell

1. Opsonin deposits on the target surface

2. Specific phagocyte receptor binds the coating

3. Receptor engagement favors ingestion

Slide 8 · Two principal opsonins in this context

IgG and C3b provide distinct receptor-binding routes

IgG

Fab binds cell antigen; Exposed Fc is recognized

Phagocyte Fc receptors

C3b

Complement cleavage product deposited on a cell

Complement receptors on phagocytes

Slide 9 · The IgG molecule performs two different tasks

Antigen specificity and effector recruitment reside in different IgG regions

1

Fab arms recognize antigen

2

IgG remains attached to cell

3

Fc region faces effector cells

4

Fc receptor mediates recognition

Slide 10 · Fc receptor engagement links antibody to ingestion

Fc receptors recognize Fc portions of relevant IgG subclasses

Targetcell

Phagocyte

Fc recognition

Engulfment →phagolysosome

IgG-coated cell

1. IgG coats an erythrocyte or other host cell

2. A macrophage or neutrophil binds the IgG Fc region

3. Target recognition precedes internalization

Slide 11 · Direct IgG opsonization: Causal sequence

Binding alone is not the entire destructive process

1

Antibody binds cell-surface antigen

2

Exposed Fc engages phagocyte

3

Cytoskeleton supports engulfment

4

Internalized cell is destroyed

Slide 12 · Classical complement can begin on an antibody-coated cell

IgM is important for complement activation, not direct FcγR uptake

1

IgG or IgM binds cell surface

2

Classical complement pathway is activated

3

C3 undergoes cleavage

4

C3b deposits on the cell

Slide 13 · C3b deposition changes the target surface

Complement amplifies recognition of an antibody-tagged target

Targetcell

Phagocyte

C3b receptor

Engulfment →phagolysosome

C3b coating

1. Complement activation yields C3 fragments

2. C3b becomes associated with the coated cell

3. Complement receptors bind deposited C3b

Slide 14 · Complement receptors complete the bridge

C3b promotes phagocytosis by neutrophils and macrophages

1

Cell surface carries C3b

2

Phagocyte complement receptor binds C3b

3

Cell is engulfed

4

Intracellular digestion clears the target

Slide 15 · IgG and IgM: Not equivalent receptor routes

Separate antibody recognition from complement-mediated opsonization

IgG-coated cell

Direct phagocyte Fc recognition is possible

Can also activate complement

IgM-coated cell

Does not use the IgG-specific Fc receptor route

Can activate classical complement and promote C3b coating

Slide 16 · Dual coating creates parallel recognition routes

The surface can display antibody and complement simultaneously

Targetcell

Phagocyte

Fc + C3b receptors

Engulfment →phagolysosome

IgG coating + C3b deposition

1. IgG exposes Fc to Fc receptors

2. Complement deposits C3b on the same target

3. Either route may support phagocyte attachment

Slide 17 · An opsonin is not a toxin

Distinguish a recognition tag from a membrane-disrupting effector

Opsonization

Marks a particle for receptor-dependent ingestion

Requires a functional responding phagocyte

Membrane attack

Damages the target membrane directly

Causes osmotic cell lysis

Slide 18 · Why recognition can be more efficient after coating

The receptor sees the deposited opsonin rather than the underlying antigen

1

Uncoated target is poorly engaged

2

Opsonin presents receptor ligand

3

Phagocyte establishes stable contact

4

Ingestion becomes more effective

Slide 19 · Professional phagocytes in this mechanism

Both cell classes participate in antibody-dependent effector responses

Macrophage

Recognizes IgG Fc and deposited C3b

Engulfs and degrades antibody-coated cells

Neutrophil

Expresses opsonin-binding receptors

Can ingest targets and release injurious mediators

Slide 20 · Phagocytosis has three major stages

The generalized sequence also applies to ingestion of antibody-coated targets

1

Recognition and attachment

2

Engulfment → phagosome

3

Killing and degradation

Sequential causal pathway

Slide 21 · Attachment precedes membrane remodeling

Physical proximity is not equivalent to receptor-mediated uptake

Targetcell

Phagocyte

Fc recognition

Engulfment →phagolysosome

IgG-coated cell

1. Surface ligand is recognized by a phagocytic receptor

2. Receptor engagement stabilizes cell–target contact

3. Signals initiate membrane and actin changes

Slide 22 · Receptor signaling reorganizes actin

Cytoskeletal remodeling drives engulfment

1

Target binds receptor

2

Intracellular signals are integrated

3

Actin polymerization reshapes membrane

4

Cell extends around the target

Slide 23 · Engulfment creates an intracellular compartment

The ingested target is moved into a membrane-bound vacuole

1

Pseudopods surround attached particle

2

Membranes close around the particle

3

Phagosome becomes a sealed vesicle

Sequential causal pathway

Slide 24 · Phagosome and lysosome are separate initially

Fusion connects uptake to degradation

Phagosome

Newly formed vesicle around ingested material

Contains the engulfed target

Lysosome / granule

Organelle containing hydrolytic contents

Supplies enzymes to the phagocytic compartment

Slide 25 · Formation of the phagolysosome

The phagolysosome is the destructive compartment

1

Phagosome contains target

2

Lysosomal granule fuses with phagosome

3

Lysosomal contents enter the compartment

4

Enzymatic degradation proceeds

Slide 26 · Intracellular destruction employs multiple effectors

Digestion is not caused by receptor binding alone

Lysosomal enzymes

Digest proteins and other internalized material

Work within the phagolysosome

Reactive intermediates

Reactive oxygen and nitrogen species contribute

Support killing and degradation after activation

Slide 27 · Compartmentalization protects the phagocyte

Normal phagocytosis separates potent effectors from the cytoplasm

1

Phagocyte produces potentially injurious molecules

2

Destructive activity is concentrated in granule-derived compartments

3

Ingested material meets destructive contents

4

Host-cell structures are relatively protected

Slide 28 · Respiratory burst: An oxidative mechanism

The respiratory burst is a phagocyte effector function

1

Phagocyte activation

2

NADPH oxidase assembly

3

Oxygen converted to superoxide

4

Reactive oxygen species contribute to microbial killing

Slide 29 · Phagocytosis and extracellular release differ

Where effectors are released governs collateral tissue injury

Intracellular pathway

Target is ingested into a phagolysosome

Contents degrade within the cell

Extracellular release

Activated leukocytes release enzymes or ROS outside

Surrounding tissues may be injured

Slide 30 · Complete cell depletion requires several linked events

An intact molecular and cellular chain produces antibody-dependent depletion

1

Antigen-specific antibody

2

Opsonin deposition on cell

3

Phagocyte receptor engagement

4

Ingestion and degradation

Slide 31 · The membrane attack complex is a distinct route

This is direct complement-mediated lysis, not engulfment

Targetcell

MACmembrane lesion

Osmotic lysis

1. Complement activation can proceed to terminal components

2. Membrane attack complex disrupts a lipid bilayer

3. Osmotic lysis destroys susceptible targets

Slide 32 · C3b opsonization versus terminal complement attack

Complement is capable of both tagging and lysing cells

C3b branch

Deposits a receptor ligand on cell surface

Enhances phagocyte uptake

Membrane attack branch

Assembles a membrane-disrupting complex

Induces target-cell lysis

Slide 33 · ADCC recognizes IgG but avoids engulfment

Antibody-dependent cellular cytotoxicity is a separate pathway

Targetcell

NK cell

Fc recognition

Target killing(no engulfment)

IgG-coated cell

1. Target cell carries surface-bound IgG

2. Effector Fc receptor engages IgG

3. Target is killed without phagocytosis

Slide 34 · Which cells can execute ADCC?

Effector identity and outcome both matter

Natural killer cell

Principal effector highlighted by Robbins

Fc-dependent recognition leads to target killing

Macrophage

Can also mediate antibody-dependent killing

Fc receptor involvement does not guarantee phagocytosis

Slide 35 · ADCC is not synonymous with phagocytosis

The presence or absence of engulfment is decisive

Phagocytosis

Target is internalized

Destruction inside a phagolysosome

ADCC

Effector binds an antibody-coated target

Killing occurs without ingestion

Slide 36 · What is known about ADCC in disease?

ADCC as a possible antibody-dependent destructive mechanism

NK cells and macrophages can bind the Fc fragment of cell-bound IgG

Its contribution to common hypersensitivity disorders is uncertain

Do not attribute a specific clinical cytopenia predominantly to ADCC without evidence

Slide 37 · Clinical theme: Loss of circulating blood cells

Different blood-cell targets lead to different clinical syndromes

1

Antibody recognizes blood-cell antigen

2

IgG and/or complement coats cells

3

Effector pathway destroys coated cells

4

Circulating cell population decreases

Slide 38 · Incompatible transfusion: Antigenic mismatch

Pre-existing antibody makes target recognition possible

1

Donor red cell bears mismatched blood-group antigen

2

Recipient has preformed reactive antibody

3

Antibody coats transfused erythrocytes

4

Complement and phagocyte pathways can destroy cells

Slide 39 · Transfusion reaction: The target is the donor cell

The donor–recipient antigen difference initiates the reaction

Target

Transfused red blood cell

Bears antigen against which recipient antibody reacts

Effector context

Recipient antibody and complement

Can promote antibody-dependent cell destruction

Slide 40 · Fetal and neonatal hemolysis: Maternal antibody

Transplacental IgG establishes the link between maternal immunity and fetal injury

1

Maternal–fetal erythrocyte antigen difference

2

Maternal antierythrocyte IgG

3

IgG crosses the placenta

4

Fetal erythrocytes undergo antibody-dependent destruction

Slide 41 · Why the IgG class matters in fetal injury

Do not equate every maternal antibody response with fetal hemolysis

Maternal IgG

Can cross the placenta

May bind fetal red-cell antigens

Complement-associated effects

Can occur after antibody deposition

The central source claim is IgG-mediated fetal-cell destruction

Slide 42 · Autoimmune hemolytic anemia: A self-cell target

An autoantibody can make a normal host cell a target

1

Autoantibody recognizes erythrocyte

2

Host red cell becomes antibody-coated

3

Destructive effector pathway is recruited

4

Red-cell population is depleted

Slide 43 · Autoimmune thrombocytopenia: Target shifts to platelets

Mechanism is shared with other immune cytopenias; The target differs

1

Antibody reacts with platelet surface

2

Platelet becomes an opsonized target

3

Antibody-dependent removal follows

4

Platelet numbers decline

Slide 44 · Immune agranulocytosis: Granulocyte depletion

Blood-cell specificity determines which lineage is depleted

1

Antibody targets host granulocytes

2

Granulocytes are marked for destruction

3

Antibody-dependent removal occurs

4

Granulocyte population decreases

Slide 45 · Three autoimmune cytopenias: One mechanism

The clinical label follows the affected cell lineage

Target cell

Named disorder

Core antibody-mediated event

Erythrocyte

Autoimmune hemolytic anemia

Antibody-dependent RBC destruction

Platelet

Autoimmune thrombocytopenia

Antibody-dependent platelet destruction

Granulocyte

Immune agranulocytosis

Antibody-dependent granulocyte destruction

Slide 46 · Drug-associated injury: Drug–protein complex

The drug creates an acquired antigenic target on a host cell

1

Drug associates with host membrane protein

2

A drug–protein antigenic complex is formed

3

Antibody recognizes the complex

4

Coated blood cell may be destroyed

Slide 47 · Drug-associated injury: Altered epitope

Antigen modification can change recognition without changing cell lineage

1

Drug modifies antigen conformation

2

A new antigenic epitope becomes exposed

3

Antibody recognizes the altered determinant

4

Blood-cell damage may follow

Slide 48 · Two drug mechanisms are not identical

Both mechanisms produce antibody reactivity toward a host-cell surface

Drug–protein complex

Drug binds a cell-membrane protein

Antibody recognizes the composite antigen

Drug-altered antigen

Drug changes antigen conformation

Antibody recognizes a newly generated epitope

Slide 49 · Circulating cells versus basement membranes

Antigen anchoring changes what an effector cell can do

Circulating blood cell

Can be physically engulfed after opsonization

Depletion dominates the explanatory pathway

Fixed matrix

Cannot be removed like an intact circulating cell

Inflammatory mediator release dominates injury

Slide 50 · Complement can recruit inflammatory leukocytes

Complement mediates both opsonization and inflammatory recruitment

1

Antibody deposits on fixed tissue

2

Complement cascade is activated

3

C5a drives leukocyte chemotaxis

4

Leukocytes accumulate near the target

Slide 51 · Anaphylatoxins amplify vascular reactions

The same complement response can change leukocyte traffic and vascular leakage

C3a

Generated during complement activation

Contributes to increased vascular permeability

C5a

Chemotactic and anaphylatoxic activity

Promotes recruitment and permeability

Slide 52 · Fc and complement receptors can activate leukocytes

Surface recognition can trigger inflammation without engulfing the fixed tissue

1

Tissue-bound antibody / complement

2

Leukocyte Fc and C3b receptor engagement

3

Leukocyte activation

4

Release of injurious products

Slide 53 · Why enzymes can injure extracellular matrix

Extracellular proteolysis provides a direct mechanism of tissue damage

1

Activated leukocyte releases lysosomal enzymes

2

Proteases encounter nearby tissue

3

Basement membrane, collagen, elastin or cartilage is degraded

4

Tissue integrity is impaired

Slide 54 · Reactive oxygen species contribute to tissue injury

Useful antimicrobial chemistry can be harmful when released into tissues

1

Leukocyte activation

2

Reactive oxygen intermediates generated

3

Oxidants can reach adjacent structures

4

Inflammatory tissue damage increases

Slide 55 · A third type II outcome: Receptor dysfunction

Not all type II reactions remove target cells

Cell depletion

Target cell is physically destroyed

Example: Antibody-mediated blood-cell loss

Receptor dysfunction

Antibody changes receptor signaling/function

Example: Thyroid-stimulating receptor activation in Graves disease

Slide 56 · Mechanism selection from the target context

Read the antigen location and endpoint before assigning mechanism

Antibody target

Likely dominant route

Mechanistic endpoint

Mobile cell surface

Opsonin + phagocyte receptor

Cell engulfment and clearance

Susceptible membrane

Terminal complement complex

Osmotic lysis

IgG-coated cell + cytotoxic effector

Fc-mediated ADCC

Killing without ingestion

Fixed tissue matrix

Complement + recruited leukocytes

Inflammatory damage

Slide 57 · Clinical reasoning exercise: Incompatible transfusion

A patient receives blood containing erythrocytes with an incompatible antigen.

Preformed recipient antibody reacts with those donor cells.

Which target is coated, and how can it be destroyed?

Mechanistic task: Identify target → opsonin → effector

Slide 58 · Reasoning: Transfusion incompatibility

The antibody is already present before exposure to the donor target

1

Donor erythrocyte carries mismatched antigen

2

Recipient antibody attaches

3

Complement and/or phagocytes are engaged

4

Antibody-dependent red-cell destruction results

Slide 59 · Clinical reasoning exercise: Fetal anemia

Maternal and fetal erythrocytes differ antigenically.

The mother produces antierythrocyte IgG that enters fetal circulation.

What is the causal link between maternal immune response and fetal cell loss?

Mechanistic task: Track compartment crossing before the effector pathway

Slide 60 · Reasoning: Transplacental antibody

IgG transfer enables injury across the maternal–fetal interface

1

Maternal antierythrocyte IgG

2

Placental passage of IgG

3

IgG coats fetal erythrocytes

4

Antibody-dependent fetal RBC destruction

Slide 61 · Clinical reasoning exercise: Drug-associated cytopenia

A drug binds a membrane-associated protein on circulating blood cells.

Antibody reactivity emerges toward the drug–protein complex.

Is the antibody necessarily directed against the unchanged native protein?

Mechanistic task: Distinguish an acquired epitope from a constitutive self-antigen

Slide 62 · Reasoning: Acquired drug-associated target

An acquired surface determinant can make a host cell vulnerable

1

Drug modifies the antigenic surface

2

Antibody recognizes a drug-related determinant

3

Blood cell acquires an opsonin coating

4

Immune destruction becomes possible

Slide 63 · Interpretation check: Identify the endpoint

1. C3b on a cell recognized by a phagocyte → ingestion

2. MAC disrupts lipid bilayer → osmotic lysis

3. NK cell binds IgG-coated target → killing without uptake

4. Complement at basement membrane → leukocyte-mediated inflammation

Slide 64 · Synthesis: Follow antigen → effector → endpoint

An antibody–target interaction is interpreted through its downstream effector

1

Cell-surface antigen

2

Surface-bound antibody ± C3b

3

Phagocyte, complement or cytotoxic cell

4

Cell clearance, lysis or injury

Slide 65 · Core mechanisms to retain

IgG Fc receptors and C3b receptors permit uptake of opsonized cells

Engulfment and phagolysosomal degradation remove antibody-coated cells

Terminal complement attack lyses membranes; ADCC kills without ingestion

Target localization explains the distinction between cytopenia and inflammation

Slide 66 · Source reference

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

Chapter 6: Antibody-mediated (type II) hypersensitivity; opsonization and phagocytosis.

Chapter 3: Phagocytosis and intracellular destruction of microbes and debris.