Diapedesis vs. Phagocytosis: Distinct Leukocyte Functions
These are two separate processes in leukocyte biology:
Diapedesis (Transmigration / Extravasation)
- Definition: Movement of leukocytes from blood vessel lumen into tissue.
- Steps: Margination → rolling (selectins) → activation (chemokines) → firm adhesion (integrins: LFA-1/ICAM-1, VLA-4/VCAM-1) → transmigration (PECAM-1, CD99, JAMs) → chemotaxis to injury site.
- Occurs in: Post-capillary venules (primary site).
Phagocytosis
- Definition: Engulfment and internalization of particles (>0.5 µm) into a phagosome.
- Cells: Neutrophils, monocytes/macrophages, dendritic cells (professional phagocytes).
- Steps: Recognition (opsonins: IgG, C3b; PAMPs) → engulfment → phagosome formation → fusion with lysosome → phagolysosome → oxidative burst (NADPH oxidase) + enzymatic killing → exocytosis of residue.
Key Distinction
| Feature | Diapedesis | Phagocytosis |
|---|---|---|
| Direction | Intravascular → extravascular | Extracellular → intracellular |
| Primary actors | All leukocytes (neutrophils, monocytes, lymphocytes) | Professional phagocytes (neutrophils, macrophages, DCs) |
| Molecular drivers | Selectins, integrins, chemokines | FcγR, complement receptors, PRRs |
| Outcome | Cell relocation | Pathogen destruction / antigen presentation |