It's axiomatic that blood is a good thing, and that not having enough of it is a bad thing. This is true as far as it goes, but there's mounting evidence that the picture is not that simple.
When patients who are critically ill are sent to intensive care units, their blood haemoglobin levels are routinely checked. If the levels drop too far below normal, blood transfusions are often given to bring levels up.
But this common-sense response may be counterproductive. It's been found that patients given blood transfusions in ICUs are at greater risk of dying than equally ill people who don't receive extra blood. Doctors speculate that it may be because having someone else's blood in your veins puts stress on the immune system (JAMA, 2002; 288: 1499-507).