Chemotaxis of Leukocytes

Slide 1 · Chemotaxis of Leukocytes

Chemical guidance, receptor signaling, cell migration and inflammatory patterns

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Define chemotaxis and distinguish it from leukocyte adhesion and diapedesis.

Identify microbial and host-derived chemoattractants and their receptors.

Explain GPCR-to-cytoskeleton signaling and polarized movement.

Interpret time- and stimulus-dependent patterns of leukocyte recruitment.

Connect chemotaxis with host defense and its clinical limitations.

Slide 3 · Chemotaxis is directional leukocyte locomotion

Chemotaxis means movement along a chemical concentration gradient.

Leukocytes in tissues move toward a source of infection or cell injury.

Guidance differs from simply increasing the number of circulating cells.

Leukocyte

Increasing attractant

Source of tissue injury

Schematic

The gradient supplies orientation; Cellular motility supplies displacement.

Slide 4 · Chemotaxis occurs after leukocyte extravasation

Bloodborne leukocytes first interact with activated vascular endothelium.

They then traverse the endothelial barrier and basement membrane.

Directional tissue migration brings them to the offending stimulus.

Rolling and adhesion

Transendothelial migration

Chemotaxis within tissue

Chemotaxis is a tissue-migration phase within a broader recruitment process.

Slide 5 · Leukocyte recruitment is a multistep process

Each stage permits the next; Different molecular systems predominate.

Margination and rolling

Chemokine-induced integrin activation

Firm adhesion and diapedesis

Tissue chemotaxis

Defect at any upstream stage can reduce delivery to an injury site.

Slide 6 · Rolling is not the same as chemotaxis

Selectins mediate initial transient endothelial contacts.

Integrins stabilize attachment before vascular exit.

Chemoattractants guide cellular movement in the interstitium.

Rolling

Transient adhesive contacts under blood flow.

Chemotaxis

Directional locomotion along a tissue chemical gradient.

Terminology reflects distinct mechanisms and anatomic compartments.

Slide 7 · A chemical gradient creates spatial information

The source of injury or microbes contributes chemoattractant molecules.

Concentration differences orient leukocyte movement.

The cell migrates toward the stronger chemotactic stimulus.

Migrating leukocyte

Higher concentration

Chemoattractant source

Schematic

The orientation cue is chemical rather than mechanical.

Slide 8 · Sources of chemoattractants cluster at injured tissue

Microbial products may themselves attract leukocytes.

Injured tissue and host inflammatory pathways also generate mediators.

Directional movement concentrates leukocytes where defense is required.

Microbes or tissue injury

Local chemoattractant signals

Leukocyte accumulation

Injury generates a local focus for recruitment.

Slide 9 · Chemoattractants have exogenous and endogenous origins

Exogenous cues include bacterial N-formyl peptides and certain bacterial lipids.

Endogenous cues include chemokines, complement fragments, and LTB4.

Both origins can contribute to the same acute inflammatory focus.

Exogenous

Compounds supplied by microbes.

Endogenous

Host-generated inflammatory mediators.

Multiple cues may coexist at one injured site.

Slide 10 · Bacterial N-formyl peptides provide microbial guidance

Bacterial products contain peptides with terminal N-formylmethionine.

These molecules function as exogenous chemoattractants.

They attract leukocytes toward the microbial source.

Phagocyte

Microbial peptide gradient

Bacterial products

Schematic

The microbe itself can contribute a directional cue.

Slide 11 · Certain bacterial lipids also serve as cues

Not every chemotactic signal is a cytokine or peptide.

Some bacterial lipids are identified as exogenous attractants.

Origin, not molecular class alone, separates endogenous from exogenous.

Bacterial peptides

N-formylmethionine-containing products.

Bacterial lipids

Additional exogenous attractant class.

Exogenous attraction is not limited to proteins.

Slide 12 · Three host mediator groups dominate the core example

Host class

Named example

Major teaching role

Chemokines

IL-8 / CXCL8

Leukocyte attraction

Complement

C5a

Chemotactic recruitment

AA metabolite

LTB4

Attraction and activation

Slide 13 · Chemokines are cytokines specialized for cell guidance

Chemokines are small proteins with leukocyte-attracting actions.

Different chemokines recruit different leukocyte subsets.

Chemokines also activate leukocytes and promote integrin affinity.

Local chemokine synthesis

Binding to leukocyte receptors

Adhesion activation and chemotaxis

Chemokines link cellular selectivity with directional recruitment.

Slide 14 · CXCL8 is a major chemokine for neutrophils

IL-8 is also designated CXCL8 in the chemokine nomenclature.

CXCL8 prominently activates and recruits neutrophils.

Its effect on monocytes and eosinophils is comparatively limited.

Neutrophil

CXCL8 gradient

Inflamed tissue

Schematic

The cell type recruited depends in part on the chemokine identity.

Slide 15 · Activated macrophages can generate CXCL8

Activated macrophages produce chemokines in response to inflammation.

Endothelial cells and additional cell types can also secrete CXCL8.

Locally produced chemokine signals help attract circulating neutrophils.

Inflammatory recognition

Macrophage / endothelial activation

CXCL8-driven recruitment

Sentinel and vascular cells help establish the chemotactic environment.

Slide 16 · TNF and IL-1 amplify chemokine recruitment

Microbial products and inflammatory cytokines induce CXCL8 production.

TNF and IL-1 are prominent cytokine inducers of CXCL8.

An early tissue signal can thus stimulate further leukocyte entry.

Microbe and inflammatory cytokines

CXCL8 expression

Neutrophil recruitment

Inflammatory signaling amplifies recruitment without changing the cell source.

Slide 17 · Endothelial proteoglycans display chemokines

Chemokines bind to proteoglycans on endothelial surfaces.

This displays local chemokine signals to rolling leukocytes.

Presentation of chemokines helps activate adhesive integrins.

Postcapillary venule

Chemokines retained on the vascular surface stimulate rolling cells.

Schematic

Surface display makes a soluble signal locally accessible.

Slide 18 · Chemokines produce a high-affinity integrin state

Rolling leukocytes initially carry low-affinity integrins.

Endothelium-bound chemokines activate the adherent leukocyte.

Integrins shift toward higher affinity for endothelial ligands.

Rolling leukocyte

Chemokine receptor engagement

Higher-affinity integrins

Chemokine sensing can convert loose contact into firm arrest.

Slide 19 · Chemokine families help explain leukocyte selectivity

Class

Representative

Prominent context

CXC

CXCL8 (IL-8)

Neutrophil chemotaxis

CC

CCL2; CCL11

Monocytes; Eosinophils

C

XCL1

Lymphocytes

CX3C

CX3CL1

Monocytes and T cells

Slide 20 · CXC chemokines commonly recruit neutrophils

CXC chemokines have one amino acid between the first two conserved cysteines.

A functional subset acts primarily on neutrophils.

CXCL8 is the major example highlighted in acute inflammation.

CXC family structure

Receptor-mediated activation

Neutrophil chemotaxis

Sequence classification can correlate with cell-type recruitment.

Slide 21 · CC chemokines often attract mononuclear cells

CC chemokines contain adjacent first two conserved cysteines.

Examples include CCL2 (MCP-1) and CCL3 (MIP-1α).

Members overlap in activity but tend to attract monocytes, lymphocytes, and selected granulocytes.

CCL2 / MCP-1

Example linked to monocyte recruitment.

CCL3 / MIP-1α

Member of a family with overlapping targets.

CC chemokines are generally less potent for neutrophils than CXC examples.

Slide 22 · CCL11 has marked eosinophil selectivity

Eotaxin is also called CCL11.

Eotaxin as selectively recruiting eosinophils.

The mediator helps explain eosinophil-rich allergic and helminthic responses.

CCL11 / eotaxin

Eosinophil-directed attraction

Characteristic infiltrate

A chemokine can bias the leukocyte composition at an injury site.

Slide 23 · C and CX3C chemokines broaden the recruitment repertoire

XCL1 is a C-family chemokine with relative lymphocyte specificity.

CX3CL1 (fractalkine) exists in cell-bound and soluble forms.

The bound form supports adhesion; The soluble form attracts monocytes and T cells.

XCL1 / lymphotactin

A C chemokine with lymphocyte preference.

CX3CL1 / fractalkine

Membrane-associated adhesion; Soluble chemoattraction.

Chemokine effects extend beyond neutrophil-dominant inflammation.

Slide 24 · Not all chemokines are inflammation inducible

Inflammatory chemokines are induced by microbes or injury signals.

Homeostatic chemokines maintain organized cellular positioning in tissues.

In lymphoid organs they help localize T and B lymphocytes to distinct regions.

Inflammatory

Inducible recruitment to a damaged site.

Homeostatic

Constitutive regulation of tissue architecture.

Chemotactic guidance is also a normal tissue-organizing mechanism.

Slide 25 · C5a connects complement activation to chemotaxis

Complement activation yields biologically active cleavage fragments.

C5a is a major chemotactic complement-derived mediator.

It also contributes to the inflammatory actions of complement.

Complement cascade

Generation of C5a

Leukocyte recruitment

Innate humoral activation can directly guide cellular migration.

Slide 26 · C5 convertase generates C5a during complement activation

All major complement activation pathways converge on common downstream steps.

C5 convertase cleaves C5 into C5a and C5b.

Released C5a can act on leukocytes at inflammatory sites.

Complement activation

C5 convertase cleaves C5

C5a release

C5a formation arises from proteolytic complement activation.

Slide 27 · C5a recruits multiple leukocyte populations

Cell population

C5a chemotactic action

Interpretive point

Neutrophils

Recognized target

Acute innate response

Monocytes

Recognized target

Later tissue macrophages

Eosinophils

Recognized target

Granulocyte recruitment

Basophils

Recognized target

Broad leukocyte activity

Slide 28 · C5a also amplifies inflammatory mediator release

C5a activates the lipoxygenase pathway in neutrophils and monocytes.

This promotes additional inflammatory mediator production.

The same complement fragment therefore attracts and activates recruited cells.

Complement-derived C5a

Leukocyte signaling

Additional inflammatory mediators

A chemoattractant can also amplify the downstream inflammatory program.

Slide 29 · LTB4 is a lipid-derived chemoattractant

Leukotriene B4 is derived from arachidonic acid metabolism.

It is a potent chemoattractant for neutrophils.

It also activates neutrophils after receptor engagement.

Arachidonic acid

Leukotriene B4

Neutrophil attraction

Chemoattractants include lipids as well as peptides and proteins.

Slide 30 · The lipoxygenase pathway produces leukotrienes

Lipoxygenase enzymes metabolize arachidonic acid.

5-lipoxygenase is especially prominent in neutrophils.

LTB4 is one product of the leukotriene-forming pathway.

Arachidonic acid

Lipoxygenase activity

LTB4 formation

Eicosanoid synthesis converts a membrane-derived substrate into inflammatory signals.

Slide 31 · LTB4 attracts and activates neutrophils

LTB4 promotes leukocyte chemotaxis.

It also augments aggregation and adhesion to venular endothelium.

Neutrophils may generate reactive oxygen species and release lysosomal enzymes.

Recruitment

Chemotaxis, aggregation, and adhesion.

Effector activation

ROS generation and lysosomal enzyme release.

Chemotactic signals can prime destructive effector responses.

Slide 32 · LTB4 differs from cysteinyl leukotrienes

LTB4 is strongly associated with leukocyte attraction and activation.

LTC4, LTD4, and LTE4 primarily cause bronchospasm and increased venular permeability.

Both mediator sets are leukotrienes but their emphasized actions differ.

LTB4

Neutrophil chemotaxis and activation.

LTC4 / LTD4 / LTE4

Smooth-muscle and vascular-permeability effects.

Do not treat all leukotrienes as mechanistically interchangeable.

Slide 33 · Lipoxins counter neutrophil recruitment

Lipoxins are also products of arachidonic acid metabolism.

They suppress neutrophil chemotaxis and endothelial adhesion.

Their activity illustrates the active limitation of inflammatory recruitment.

Leukotrienes

Promote acute leukocyte recruitment.

Lipoxins

Inhibit neutrophil chemotaxis and adhesion.

Related lipid mediator pathways can have opposing inflammatory effects.

Slide 34 · Distinct chemoattractants converge on GPCR signaling

Chemokines, C5a, LTB4, and bacterial products act through their specific receptors.

The relevant chemotactic receptors belong to the seven-transmembrane GPCR class.

Different ligands therefore converge on a shared signaling logic.

Distinct ligand–receptor pairs

Seven-transmembrane GPCRs

Movement-linked signaling

A common receptor architecture supports multiple chemotactic inputs.

Slide 35 · Chemotactic receptors span the membrane seven times

The receptor resides in the leukocyte plasma membrane.

The extracellular signal is translated into intracellular G-protein signaling.

A seven-transmembrane GPCR is a receptor type, not seven separate receptors.

Chemoattractant

Leukocyte plasma membrane

G-protein signaling → second messengers

Seven-transmembrane receptor • schematic

Ligand recognition is converted into changes inside the leukocyte.

Slide 36 · Receptor activation engages intracellular second messengers

Ligand binding activates its cognate surface receptor.

Intracellular second messengers relay the received signal.

These signals reorganize the actin–myosin cytoskeleton.

Chemoattractant binding

GPCR second-messenger signaling

Cytoskeletal reorganization

Chemotaxis is an actively signaled cellular response—not passive drift.

Slide 37 · Actin polymerizes at the leading edge

A leukocyte develops a spatially defined front and rear during migration.

Actin polymerization occurs at the leading edge.

The extended front advances toward the stronger attractant cue.

Direction of higher chemoattractant

Rear: Myosin-associated contraction

Front: Actin polymerization and protrusion

Forward protrusion is driven by regulated cytoskeletal remodeling.

Slide 38 · Myosin filaments localize toward the cell rear

Chemotactic receptor signaling also reorganizes myosin.

Myosin filaments are localized predominantly toward the trailing region.

This organization contributes to movement of the cell body.

Direction of higher chemoattractant

Rear: Myosin-associated contraction

Front: Actin polymerization and protrusion

Front-directed actin and rear-positioned myosin cooperate.

Slide 39 · Front–rear polarity enables persistent directionality

Actin-rich protrusion develops at the front.

Myosin-associated contraction is located toward the rear.

The asymmetry permits net migration rather than undirected shape change.

Direction of higher chemoattractant

Rear: Myosin-associated contraction

Front: Actin polymerization and protrusion

Directional movement depends on spatial coordination of the cytoskeleton.

Slide 40 · Filopodia extend toward the attractant source

The leading edge produces cytoplasmic extensions termed filopodia.

These extensions lead leukocyte movement toward increasing signal.

The cell body follows the advancing edge.

Polarized cell

Increasing chemotactic cue

Site of inflammatory stimulus

Schematic

Protrusion pulls the cell toward the injury focus.

Slide 41 · Motility requires repeated protrusion and translocation

Chemotactic migration is a coordinated sequence of receptor signals and cell-shape change.

Ligand sensing

Actin-rich front extends

Myosin localized posteriorly

Cell moves toward source

The intracellular machinery translates a chemical gradient into locomotion.

Slide 42 · Chemotaxis links two kinds of information

The concentration gradient indicates where to move.

GPCR-triggered cytoskeletal remodeling supplies the mechanism of movement.

Both elements are needed to explain directional accumulation in tissues.

Directional information

Local differences in attractant signal.

Mechanical output

Actin extension and rearward myosin localization.

A gradient alone does not move a cell; Receptor-driven remodeling does.

Slide 43 · Chemokines connect vascular adhesion with tissue guidance

On endothelial surfaces, chemokines raise leukocyte integrin affinity.

After vascular exit, chemokines guide movement through tissues.

One signal class therefore participates at more than one recruitment stage.

At the endothelial interface

Converts integrins to a high-affinity state.

Inside interstitial tissue

Provides chemotactic guidance.

Location determines the dominant effect of the same signaling class.

Slide 44 · Selectins support the first adhesive contacts

P-selectin and E-selectin participate in leukocyte rolling.

Rolling brings leukocytes into repeated contact with endothelial chemokines.

Selectins are not the principal molecules of firm attachment.

Postcapillary venule

Selectin-mediated rolling under flow

Schematic

Initial contact increases access to endothelial activation signals.

Slide 45 · Integrin activation converts rolling into arrest

Resting leukocyte integrins are generally in a lower-affinity state.

Chemokine receptor activation increases integrin affinity.

Interaction with endothelial ligands produces firm adhesion and spreading.

Lower-affinity integrins

Rolling leukocyte is not fully arrested.

Higher-affinity integrins

Leukocyte binds firmly to activated endothelium.

Inside-out activation of adhesion receptors precedes transmigration.

Slide 46 · ICAM-1 and VCAM-1 support stable adhesion

Activated endothelium expresses integrin ligands including ICAM-1 and VCAM-1.

Leukocyte LFA-1 and VLA-4 integrins bind their corresponding ligands.

High receptor affinity and endothelial ligand display produce stable contact.

Postcapillary venule

Integrin–ligand binding arrests leukocytes.

Schematic

Firm adhesion is an interaction between two activated cellular surfaces.

Slide 47 · CD31 is involved in endothelial transmigration

Leukocytes pass mainly through postcapillary venules in typical acute inflammation.

CD31 (PECAM-1) participates at endothelial intercellular junctions.

This step is also termed diapedesis.

Postcapillary venule

PECAM-1 participates at endothelial cell junctions.

Schematic

Diapedesis follows firm adhesion and precedes full tissue chemotaxis.

Slide 48 · Leukocytes then cross the basement membrane

Following endothelial transit, leukocytes must traverse the vessel basement membrane.

Collagenase secretion is a proposed means of enabling this passage.

The basement membrane is repaired after leukocyte transit.

Endothelial crossing

Basement-membrane passage

Extravascular accumulation

The vascular wall presents sequential structural barriers.

Slide 49 · Recruitment culminates in migration toward injury

The dominant molecular mechanisms differ at successive stages of recruitment.

Transient rolling: Selectins

Firm adhesion: Integrins

Junctional transit: CD31

Tissue chemotaxis: GPCRs

Recruitment delivers phagocytic cells to the site requiring defense.

Slide 50 · Reaching the target is not the same as killing it

Chemotaxis delivers leukocytes to infected or injured tissue.

Recognition receptors then bind microbes and other offending material.

Phagocytosis and intracellular destruction constitute subsequent effector functions.

Chemotaxis to the focus

Recognition and engulfment

Phagolysosomal destruction

Recruitment and microbe removal are sequential but distinct processes.

Slide 51 · Neutrophils usually predominate early

Neutrophils are the usual major infiltrating cells during early acute inflammation.

Robbins gives a typical predominance during the first 6–24 hours.

This is a common pattern, not a universal rule.

6–24 hours

Often neutrophil-rich.

Beyond first day

Relative composition can shift.

Time and stimulus jointly shape infiltrate composition.

Slide 52 · Several features favor rapid neutrophil recruitment

Neutrophils are abundant in the circulation relative to other leukocytes.

They respond rapidly to chemokines.

They may adhere strongly to rapidly induced endothelial P- and E-selectin.

Availability in blood

Fast chemokine responses

Efficient endothelial capture

Early neutrophil predominance has multiple contributing reasons.

Slide 53 · Monocytes gain prominence at 24–48 hours

In many acute reactions, monocytes increasingly replace neutrophils after the first day.

This transition generally within 24–48 hours.

Recruited monocytes differentiate into tissue macrophages.

6–24 hours

Neutrophils commonly dominate.

24–48 hours

Monocytes increasingly accumulate.

A later macrophage-rich field can reflect evolving inflammation.

Slide 54 · Neutrophil apoptosis contributes to the cellular shift

Extravascular neutrophils are relatively short-lived.

Many undergo apoptosis within days after tissue entry.

Short survival contributes to their replacement by other inflammatory cells.

Early phase

Rapid influx of neutrophils.

Later phase

Apoptosis reduces accumulated neutrophils.

Loss of existing cells matters as well as recruitment of new ones.

Slide 55 · Macrophages can persist during prolonged inflammation

Macrophages derived from monocytes generally survive longer in tissues.

Macrophages may proliferate locally within an inflammatory focus.

They can become dominant when an inflammatory reaction persists.

Neutrophil

Short-lived and rapidly acting.

Macrophage

Longer-lived, potentially locally proliferative.

Persistence of macrophages contributes to later tissue responses.

Slide 56 · Cellular composition reflects both stimulus and duration

The common neutrophil-to-monocyte sequence is a useful baseline.

Different infections and immune responses can alter that pattern.

Infiltrate interpretation requires attention to the suspected cause.

Temporal influence

Cell recruitment and survival vary with time.

Etiologic influence

Distinct stimuli recruit different leukocyte types.

An inflammatory differential is mechanistic evidence, not a precise timestamp.

Slide 57 · Pseudomonas can sustain a neutrophilic infiltrate

Certain bacterial infections maintain continuing neutrophil influx for days.

Robbins specifically names Pseudomonas as an example.

A persistent neutrophil-rich response can therefore outlast the earliest phase.

Clinical observation

Neutrophil-rich tissue several days after infection

Interpretation

Ongoing neutrophil recruitment may be stimulus-dependent.

Teaching vignette — illustrative, not patient data

Do not automatically label prolonged neutrophilic inflammation as atypical.

Slide 58 · Some viral infections recruit lymphocytes early

A lymphocytic infiltrate can be present early in certain viral infections.

This deviates from the usual initial neutrophil-dominant pattern.

Interpret the cells in the context of the infectious stimulus.

Clinical observation

Early inflammation with a lymphocytic infiltrate

Interpretation

A viral stimulus is one plausible explanation.

Teaching vignette — illustrative, not patient data

A cell type is informative but not by itself etiologically diagnostic.

Slide 59 · Eosinophils may dominate allergic and helminthic responses

Eosinophils can be the prominent leukocyte population in helminth infection.

Allergic reactions can also generate eosinophil-rich inflammation.

Chemokine selectivity, including eotaxin, helps explain this bias.

Clinical observation

Eosinophil-rich inflammatory focus

Interpretation

Consider allergy or helminthic infection among causes.

Teaching vignette — illustrative, not patient data

Stimulus-specific chemotaxis reshapes the inflammatory-cell distribution.

Slide 60 · Hypersensitivity may favor mononuclear infiltrates

Some hypersensitivity reactions favor activated lymphocytes.

Macrophages and plasma cells may also predominate.

The infiltrate therefore reflects the associated immune response.

Clinical observation

Lymphocytes, macrophages, plasma cells

Interpretation

A hypersensitivity reaction is a relevant context.

Teaching vignette — illustrative, not patient data

Not all inflammatory reactions are neutrophil rich.

Slide 61 · An infarct provides a temporal histopathology example

An inflammatory reaction after myocardial ischemic necrosis.

Early images show neutrophilic infiltrates and congested vessels.

Later images show a mononuclear cellular infiltrate.

Early after injury

Neutrophils and congestion.

Later response

Increasing mononuclear cells.

The example explains a sequence; It is not a diagnostic time calculator.

Slide 62 · An infiltrate alone cannot precisely date an injury

Robbins provides approximate kinetics for typical acute inflammation.

Persistent bacterial stimulation and distinct immune responses produce exceptions.

Interpret patterns together with morphology and clinical circumstances.

Helpful observation

Relative neutrophil versus monocyte predominance.

Important limitation

Overlapping stages and stimulus-specific differences.

Temporal interpretation should remain approximate.

Slide 63 · Adhesion defects can impair antimicrobial recruitment

Inherited deficits in leukocyte adhesion molecules predispose to bacterial infections.

Defective endothelial attachment or migration limits leukocyte delivery.

Normal chemotactic signals cannot substitute for missing earlier recruitment steps.

Defective adhesion

Reduced tissue recruitment

Greater infection susceptibility

Multiple intact steps are required for effective host defense.

Slide 64 · Blocking recruitment can compromise host defense

Therapies targeting TNF or leukocyte integrins can limit harmful inflammation.

They may also reduce leukocyte entry and antimicrobial responses.

The intended anti-inflammatory benefit carries an infection-related trade-off.

Desired therapeutic effect

Reduced injurious inflammatory recruitment.

Potential consequence

Impaired defense against infectious microorganisms.

The physiologic role of recruitment explains a key treatment risk.

Slide 65 · Chemotaxis precedes phagocytic recognition

Neutrophils and macrophages move toward offending material.

Microbial recognition and attachment then initiate engulfment.

An intracellular phagosome subsequently fuses with lysosomes.

Directional recruitment

Recognition and phagocytosis

Phagolysosome formation

Arrival at the lesion allows, but does not itself perform, microbial killing.

Slide 66 · Recruited leukocytes may also damage host tissues

Leukocyte activation releases products designed to destroy microbes.

Reactive oxygen species and lysosomal enzymes can escape their normal compartments.

The same defensive mechanisms may injure nearby viable host tissue.

Recruitment and activation

Microbial destruction products

Potential collateral tissue injury

Inflammatory recruitment is protective but not intrinsically harmless.

Slide 67 · Inflammation contains mechanisms for shutting down recruitment

Inflammatory mediators generally decay when stimuli subside.

Neutrophil apoptosis limits their duration in tissues.

Lipoxins actively oppose neutrophil chemotaxis and adhesion.

Positive phase

Chemoattractants recruit effector leukocytes.

Limiting phase

Mediator clearance, apoptosis, inhibitory lipids.

Resolution depends on diminished stimulus and active restraints.

Slide 68 · Applied question: Identify the chemotactic signal

At a bacterial focus, leukocyte chemotaxis is present without naming a single cytokine.

Which exogenous and endogenous attractant families could explain this?

Distinguish the origin of the molecules from their signaling receptor class.

Clinical observation

Bacterial peptides and host inflammatory mediators

Interpretation

N-formyl peptides; CXCL8, C5a, and LTB4 are source-listed cues.

Teaching vignette — illustrative, not patient data

The correct answer contains multiple independent attractant sources.

Slide 69 · Applied question: Explain direction of movement

A neutrophil responds to a higher attractant concentration at one edge.

Which cytoskeletal changes generate migration toward this edge?

Connect receptor signaling to actin and myosin position.

Clinical observation

Chemoattractant receptor signals

Interpretation

Leading-edge actin polymerization and posterior myosin localization.

Teaching vignette — illustrative, not patient data

Chemotaxis requires polarized motility rather than passive displacement.

Slide 70 · Applied question: Interpret leukocyte predominance

A tissue specimen shows a mainly eosinophilic inflammatory infiltrate.

Which stimulus classes belong in the interpretation?

Why is “more than 48 hours old” not a sufficient explanation?

Clinical observation

Eosinophil-rich infiltrate

Interpretation

Allergic reactions or helminthic infection are relevant source-listed contexts.

Teaching vignette — illustrative, not patient data

Stimulus identity can override the stereotypic recruitment timeline.

Slide 71 · The inflammatory response is an ordered functional chain

Cell migration is indispensable but occupies only part of acute inflammation.

Injury recognized

Leukocyte recruitment

Chemotaxis and accumulation

Phagocytic clearance

Limitation and repair

Chemotactic cues couple tissue injury to the positioning of defensive cells.

Slide 72 · Mechanism checklist for leukocyte chemotaxis

Level

Essential element

Functional outcome

Signal

Microbial products; CXCL8; C5a; LTB4

Attractive gradient

Recognition

Specific GPCRs

Second-messenger activation

Motility

Front actin; Rear myosin

Directional translocation

Context

Selectins; Integrins; CD31

Effective tissue delivery

Outcome

Recruitment of suitable cells

Local defense ± injury

Slide 73 · Reference and scope

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

Chapter 3 — Inflammation and Repair

Core discussion: leukocyte chemotaxis, recruitment and evolving inflammatory infiltrates.

Supporting context: chemokines, complement, leukotrienes, lipoxins and phagocytosis.

Conceptual diagrams are original editable schematics; no histologic specimens are reproduced.