What's that big brown bag - packed full of red blood cells - for in the upper left of your chest? Sure you can live without it but why's it there in the first place and what's it doing for you? You wouldn't just be carrying round that extra 175g (for an 'average' person) just for nothing would you? While it has been known that loss of your spleen, or asplenia has some pretty strange effects, such as an increase in some blood cells, lessened response to vaccinations and an increased risk of serious infections, we haven't exactly known why. That is until now, when a German team - publishing in Nature Immunology a few weeks back - uncovered an unrecognised role for a certain kind of immune cell that specifically hides out in your spleen.