1. Paracellular Transmigration:
- Through Tight Junctions: Some immune cells can squeeze between the endothelial cells or epithelial cells that form the barrier by exploiting gaps in the tight junctions.
- Modulation of Tight Junctions: Immune cells can release signalling molecules, such as cytokines or chemokines, which promote the relaxation or disassembly of tight junctions, facilitating their passage.
2. Transcellular Transmigration:
- Direct Penetration: Certain immune cells, such as neutrophils, can physically penetrate the endothelial or epithelial cell layer by secreting proteolytic enzymes that degrade the extracellular matrix and basement membrane.
- Transcytosis: Some immune cells are taken up by endothelial cells or epithelial cells through a process called transcytosis. The immune cells are enveloped by the cell membrane and transported across the barrier.
3. Paravascular Pathways:
- Extravasation: Immune cells can adhere to the endothelium of blood vessels near the site of interest, then squeeze through the intercellular junctions and migrate into the surrounding tissue. This process is commonly observed in inflammation and immune surveillance.
4. Receptor-Mediated Transmigration:
- Integrin-Ligand Interactions: Integrins expressed on immune cells bind to specific ligands on the surface of endothelial cells or epithelial cells, helping immune cells to adhere to and traverse the barrier.
- Selectin-Ligand Interactions: Selectins expressed on immune cells interact with carbohydrate ligands on the luminal surface of endothelial cells, allowing immune cells to roll along the vessel wall and eventually migrate into the tissue.
5. Chemokine Gradients:
- Chemotaxis: Chemokines produced by tissues or immune cells can create a concentration gradient that guides immune cells towards the site of infection or inflammation. These gradients help immune cells navigate and cross tissue barriers.
6. Lymphatic System:
- Lymphatic Drainage: Immune cells can enter the lymphatic system through specialized structures called collecting lymphatics or lymphatic capillaries. The lymphatic system aids in the trafficking of immune cells between tissues and lymphoid organs.
It's important to note that immune cell extravasation and tissue migration are tightly regulated processes, balanced between the need for immune surveillance and the prevention of excessive inflammation and tissue damage. Dysregulation of these mechanisms can contribute to various immune disorders.
