## Temporal Sequence of Acute Inflammatory Cell Recruitment **Key Point:** In the classic sequence of neutrophil recruitment, **margination** (movement of neutrophils from the central axial stream to the vessel periphery) and **rolling** are not strictly sequential — they are overlapping/concurrent processes. More critically, Option A states that margination "precedes rolling AND firm adhesion via selectins and integrins," which is **incorrect in its mechanistic framing**: rolling itself is mediated by selectins and is part of the margination-to-adhesion continuum. Margination does NOT precede rolling; rather, margination and rolling occur together as part of the same early phase, both mediated by selectins. Firm adhesion via integrins follows rolling. The statement in Option A misrepresents the sequence by implying margination precedes selectin-mediated rolling, when in fact rolling IS the selectin-mediated step that occurs during/after margination. ### Correct Sequence of Neutrophil Recruitment (Robbins & Cotran, 10th ed.) | Phase | Mediators | Notes | |-------|-----------|-------| | **Margination & Rolling** | P-selectin, E-selectin, L-selectin | Concurrent; selectins mediate rolling | | **Firm Adhesion** | β2-integrins (CD11b/CD18) + ICAM-1 | Follows rolling | | **Transmigration (Diapedesis)** | PECAM-1 (CD31), VE-cadherin | Through endothelium & basement membrane | | **Chemotaxis** | C5a, IL-8 (CXCL8), LTB4, fMLP | G-protein coupled receptors on neutrophil | | **Peak Neutrophil Infiltration** | — | 24–48 hours after injury | ### Why Each Option is Correct (and Option A is the EXCEPT answer): - **Option A (INCORRECT sequence — the answer):** States margination precedes rolling via selectins. In reality, rolling IS the selectin-mediated step that accompanies margination. Margination does not precede selectin-mediated rolling; they are part of the same phase. This temporal/mechanistic description is wrong. - **Option B (Correct):** Transmigration occurs after firm adhesion and before migration into tissue — this is the correct sequence per Robbins. - **Option C (Correct):** C5a, IL-8, and LTB4 are classic chemotactic agents that bind G-protein coupled receptors on neutrophils — textbook fact (Robbins, Harrison). - **Option D (Correct):** Neutrophil recruitment does begin immediately upon injury (within minutes, endothelial activation occurs), and peak infiltration at 6–12 hours is a recognized early peak in some models (though 24–48 hours is the classic "peak" in most texts). The statement is broadly consistent with the known timeline. **High-Yield (Robbins & Cotran Pathologic Basis of Disease, 10th ed.):** The leukocyte recruitment cascade proceeds: margination → rolling (selectins) → activation → firm adhesion (integrins/ICAM-1) → transmigration (PECAM-1) → chemotaxis. Rolling and margination are concurrent selectin-dependent events, not sequential. **Clinical Pearl:** In acute bacterial pneumonia, the early neutrophilic exudate (red hepatization → grey hepatization) reflects this rapid recruitment cascade. Understanding the molecular mediators is essential for pharmacologic targeting (e.g., anti-integrin therapies in inflammatory disease). **Mnemonic:** **MR. FACT** — **M**argination/**R**olling (selectins, concurrent) → **F**irm adhesion (integrins) → **A**dhesion molecules → **C**hemotaxis → **T**issue infiltration.
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.