STEPTRACKERIMMUNOLOGY + PATHOLOGY | INNATE IMMUNITY | HIGH-YIELD REVIEW

Phagocytosis & Killing

Neutrophils leave the blood in four steps, swallow what is opsonized and kill it with oxygen radicals — each step has its own named deficiency.

FOCUS ON

Leukocyte extravasation: (1) rolling — selectins (E- and P-selectin on endothelium bind sialyl Lewis X on leukocytes); (2) tight adhesion — ICAM-1 on endothelium binds LFA-1/Mac-1 (CD18 integrins), activated by chemokines; (3) transmigration — PECAM-1 (CD31); (4) chemotaxis — C5a, IL-8, LTB₄, bacterial N-formyl peptides. Phagocytosis is helped by opsonins (IgG, C3b). Killing: NADPH oxidase makes superoxide (respiratory burst) → SOD → H₂O₂ → MPO + Cl⁻ → HOCl (bleach). Defects: LAD type 1 (CD18; no pus, delayed cord separation), CGD (NADPH oxidase; catalase ⁺ infections; abnormal DHR/NBT), MPO deficiency (mostly mild, Candida), Chédiak-Higashi (LYST; giant granules, albinism, neuropathy).

STEP

MOLECULES

DEFECT

CLUE

Margination & rolling

E-selectin, P-selectin (Weibel-Palade bodies) bind sialyl Lewis X; L-selectin on leukocytes

LAD type 2 (fucosylation defect, no sialyl Lewis X) — also Bombay blood group, ID

Rare

Tight adhesion

ICAM-1/VCAM-1 bind β2 integrins (LFA-1, Mac-1)

LAD type 1 (CD18, AR): recurrent skin/mucosal infections without pus, very high WBC, delayed umbilical cord separation, periodontitis

Steroids and catecholamines release marginated neutrophils → ↑ WBC

Transmigration (diapedesis)

PECAM-1 (CD31)

—

Post-capillary venules

Chemotaxis

C5a, IL-8 (CXCL8), LTB₄, kallikrein, N-formyl peptides

—

Neutrophils follow the gradient

Opsonization & engulfment

IgG (FcγR), C3b/iC3b (CR1, CR3)

Antibody or complement deficiency → encapsulated bacteria

Capsules resist phagocytosis unless opsonized

Phagolysosome formation

Microtubule-driven fusion of granules with phagosome

Chédiak-Higashi (LYST, AR): giant granules, partial albinism, peripheral neuropathy, pyogenic infections, bleeding, HLH

Giant inclusions on smear

Respiratory burst

NADPH oxidase: O₂ → superoxide → (SOD) → H₂O₂

CGD (X-linked gp91phox most often): infections with catalase ⁺ organisms, granulomas

Abnormal dihydrorhodamine (DHR) flow test (NBT stays colorless)

Myeloperoxidase

H₂O₂ + Cl⁻ → HOCl; gives pus its green color

MPO deficiency: usually asymptomatic; Candida infections in diabetics

Normal DHR

Oxygen-independent killing

Lysozyme, lactoferrin, defensins, major basic protein

Specific granule deficiency

—

NETs

Neutrophil extracellular traps of DNA and enzymes

Contribute to thrombosis, lupus, vasculitis

—

Resolution

Neutrophils apoptose; macrophages clear them; lipoxins, IL-10, TGF-β

Persistent stimulus → chronic inflammation, granulomas

—

Picture it — leaving the blood

Leukocyte extravasationSelectins slow the cell, activated integrins (CD18) bind ICAM-1 to stop it, PECAM-1 lets it squeeze between endothelial cells, and chemoattractants steer it to the microbe.

Schematic diagrams, not to scale.

Mechanism — the respiratory burst

O₂

→

NADPH oxidaseCGD defect

→

O₂•⁻

→

SOD → H₂O₂

→

MPO + Cl⁻ → HOClMPO defect

In CGD the neutrophil can still use H₂O₂ made by catalase-negative bacteria; catalase-positive organisms destroy their own H₂O₂ and survive.

Phagocyte defects compared

DISORDER

DEFECT

SIGNATURE

TEST

LAD 1

CD18 integrin

No pus, late cord separation

Flow CD18

CGD

NADPH oxidase

Catalase ⁺ abscesses, granulomas

DHR abnormal

Chédiak-Higashi

LYST trafficking

Giant granules, albinism, neuropathy

Smear

MPO deficiency

Myeloperoxidase

Mostly benign, Candida

DHR normal

How it's tested

COMMON CONFUSIONLAD (cells can't leave the blood → very high WBC, no pus) vs CGD (cells arrive and engulf but can't kill → abscesses, granulomas).

THE TRAPGlucocorticoids raise the neutrophil count by stopping adhesion (demargination) — not by infection.

YOUR APPROACHInfection pattern → which step failed (roll, stick, migrate, engulf, kill) → molecule → test.

If you see… think…

Rolling molecules on endothelium → E- and P-selectin

Firm adhesion pair → ICAM-1 with LFA-1 (CD18)

Umbilical cord attached at 4 weeks, WBC 50,000, no pus → LAD type 1

Transmigration molecule → PECAM-1

Liver abscess from S aureus, Aspergillus pneumonia in a boy → CGD

Test for CGD → dihydrorhodamine flow cytometry

Enzyme giving pus its green color → myeloperoxidase

Giant neutrophil granules + silvery hair + neuropathy → Chédiak-Higashi

Bleach made by neutrophils → HOCl (MPO)

Why WBC rises on prednisone → demargination