|Electron Microscope image of an Effector cell.|
As far as I can work out, this is how it appears to work: a bacteria gets into the system and the first thing to happen is that local inflammation occurs. Specific immune-system cells such as Dendritic cells - think border protection - which wander around looking for interlopers, are switched on by this inflammation and bring genetic information about the bacteria back to the local lymph node. Enter a specialized cell called an Effector Cell - think special forces - which is "trained" by the lymph node nearest the site of inflammation in the subtle art of recognizing this new invading threat. The effector cells then re-enter the blood stream to find the nearest "jump off point" so that they can get close to the site of infection and "terminate" the opposition. But how do they find the right spot to exit the artery whilst hurtling through the blood as it is pumped around the body?
Well according to Prof Ronen Alon and his co-workers Drs Ziv Shulman and Shmuel Cohen, the lining of the blood vessels is not just a passive sticky barrier, but lined by endothelial cells that have the ability to "signal" to passing Effector cells where to exit the system. But what the researchers found was that these chemical signals were held within the endothelial cells - presumably so they don't get washed away - in structures called vesicles, and that the Effector cells have literally to reach within the endothelial cells to discover that they are at the correct location. What also amazed me was that these defensive white cells have appendages like thin legs and actually crawl along the inside of the blood vessels: and as they crawl along, they "reach inside" the endothelial cells looking for the right chemical signals. It's a bit like driving down the street and stopping at each store to ask where the fire is, when you get to a cluster of stores that say "You're here", then the endothelial cells literally help the Effector cells to pass through "between the buildings" and carry out their mission!
There are still a great many puzzles to answer, but the complexity of the system is only equaled by the beauty of its organization.