May 20, 2002 | David F. Coppedge

White Blood Cells Walk to Infection on Tiny Legs

51; How do white blood cells know where to go when infection strikes?  The cells have tiny little feet and crawl like millipedes, against the blood stream, if necessary, following signals from the infection site.  When they arrive, more signals tell them where to slip through the cells of the blood vessel to get to the job.  This amazing story was reported by Science Daily based on a press release from the Weizmann Institute of Science in Rehovot, Israel (see Wiezmann Wonder Wander).  Here’s how they described this phenomenon:

How do white blood cells – immune system ‘soldiers’ – get to the site of infection or injury?  To do so, they must crawl swiftly along the lining of the blood vessel – gripping it tightly to avoid being swept away in the blood flow – all the while searching for temporary ‘road signs’ made of special adhesion molecules that let them know where to cross the blood vessel barrier so they can get to the damaged tissue.
    In research recently published in the journal Immunity, Prof. Ronen Alon and his research student Ziv Shulman of the Weizmann Institute’s Immunology Department show how white blood cells advance along the length of the endothelial cells lining the blood vessels.  Current opinion maintains that immune cells advance like inchworms, but Alon’s new findings show that the rapid movement of the white blood cells is more like that of millipedes.  Rather than sticking front and back, folding and extending to push itself forward, the cell creates numerous tiny ‘legs’ no more than a micron in length – adhesion points, rich in adhesion molecules (named LFA-1) that bind to partner adhesion molecules present on the surface of the blood vessels.  Tens of these legs attach and detach in sequence within seconds – allowing them to move rapidly while keeping a good grip on the vessels’ sides.

The press release went on to say that these legs don’t just walk.  They act as probes as they press into the epithelial tissue lining the vessels.  The force of blood actually forces them to embed their little legs into the tissue as a way to sense the location of the damaged tissue and make their way to it.  “The scientists believe that the tiny legs are trifunctional:,” the article said: “Used for gripping, moving and sensing distress signals from the damaged tissue.”

A reader found an animation of this at Harvard BioVisions.  Click on the media file labeled “Extravasation” and it will show you some of the parts and processes involved.
    It’s uncanny how the actions of these cells lacking a brain, muscles or central nervous system can act so precisely and effectively, they can be compared to multicellular organisms with all those systems.  You can almost visualize these cells like ambulance crew members or soldiers in specially designed vehicles able to cling to attachment points against the flow of traffic.  They seem so well trained and effective, it looks like they do what they do on purpose.  What a concept.

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