U01.04.015 Leukocyte extravasation

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Learning Objectives

Master the four-step sequence of Leukocyte Extravasation and identify the specific molecular interactions required for neutrophils to exit the bloodstream. Understand the clinical significance of Leukocyte Adhesion Deficiencies (LAD) and the role of chemotactic signals in guiding immune cells to the site of injury.

1. The Molecular Steps of Extravasation

Extravasation occurs predominantly at the postcapillary venules. This process transitions leukocytes from high-velocity flow to tissue-bound migration.

Step Vasculature / Stroma Ligand Leukocyte Molecule Key Notes
1. Margination & Rolling E-selectin, P-selectin, GlyCAM-1, CD34 Sialyl Lewis X, L-selectin P-selectin is released from Weibel-Palade bodies.
2. Tight Binding (Adhesion) ICAM-1 (CD54), VCAM-1 (CD106) CD11/18 integrins (LFA-1, Mac-1), VLA-4 Integrins are activated by chemokines.
3. Diapedesis (Transmigration) PECAM-1 (CD31) PECAM-1 (CD31) WBCs squeeze between endothelial cells.
4. Migration (Chemotaxis) Chemotactic Gradient Various Receptors Guided by C5a, IL-8, LTB4, and bacterial products.

2. Clinical Correlation: Adhesion Deficiencies

Molecular defects in the leukocyte’s “brakes” lead to clinical syndromes characterized by a neutrophil deficiency at the site of infection.

Condition Molecular Defect Clinical Presentation
LAD Type 1 Defective CD18 integrin subunit (LFA-1). Delayed separation of the umbilical cord; recurrent skin/mucosal infections without pus.
LAD Type 2 Decreased Sialyl Lewis X. Milder than LAD-1; associated with growth/developmental delays.

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3. Chemotactic Factors

Leukocytes move through the interstitium toward the site of injury following specific chemical signals.

Factor Origin / Type Function
C5a Complement Component Highly potent neutrophil chemoattractant.
IL-8 Cytokine (Macrophages) Recruits neutrophils to the focus of acute inflammation.
LTB4 Leukotriene (Arachidonic Acid) Promotes neutrophil chemotaxis and activation.
N-formylmethionyl Bacterial Peptides Exogenous signal found in prokaryotic proteins.

 

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High-Yield Mnemonics & Tips:

  • Rolling vs Adhesion: Selectins are for Rolling (think “S” for Slowing down). Integrins are for Adhesion (think “I” for Immobilizing).
  • LAD-1 High-Yield: The umbilical cord delay is a favorite USMLE topic. If the cells can’t “Integrate” (Integrins) into the tissue, the cord can’t be remodeled and will fall off.
  • Leukocytosis in LAD: Patients will have a high WBC count (neutrophilia) because the cells are stuck in the “highway” (vessels) and can’t pull over into the “town” (tissues).

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