How Adhesion Molecules Guide Immune Cells

Adhesion molecules are specialized proteins found on cell surfaces that help cells stick to each other and their environment.There are three main types of adhesion molecules: selectins, integrins, and members of the immunoglobulin superfamily.Selectins are like molecular hooks that catch passing cells. They have a distinctive curved shape that helps them grab their targets.The hook-like structure of selectins is perfectly designed to catch and temporarily hold passing cells, much like a velcro attachment.Integrins are fascinating molecules that can change their shape. In their inactive state, they remain folded and compact.When activated, integrins undergo a dramatic shape change, extending outward and opening up their binding site.The immunoglobulin superfamily members are chain-like proteins made up of multiple similar units.These three types of adhesion molecules work together on both immune cells and blood vessel walls, creating a sophisticated system of cellular recognition and attachment.During inflammation, the endothelial cells that line blood vessels undergo important changes.The inflamed area triggers endothelial cells to express more adhesion molecules, particularly P-selectins and E-selectins.Immune cells in the bloodstream have special carbohydrate ligands on their surface.As the immune cell approaches the vessel wall, these selectins interact with the ligands, causing the cell to slow down and begin rolling.This rolling process is dynamic, with bonds constantly forming and breaking as the cell moves along the vessel wall.At the molecular level, each selectin forms a temporary bond with its corresponding ligand, allowing for controlled cell movement.As the immune cell continues rolling, it encounters specific chemical signals called chemokines.These chemokines trigger the activation of integrins on the immune cell surface, causing them to extend and become more adhesive.The activated integrins then bind strongly to immunoglobulin molecules on the blood vessel wall.Once firmly attached, the immune cell changes shape, becoming more elongated as it prepares to migrate through the vessel wall.In a process called diapedesis or transmigration, the cell squeezes between the endothelial cells.Finally, the immune cell completes its journey through the vessel wall, entering the infected or damaged tissue where it's needed.Let's review the key steps of firm adhesion and transmigration.This completes our exploration of how immune cells leave the bloodstream to fight infection.