A universal food and alarm cue found in mammalian blood
(PHYSorg.com Biology)

Predators use the smell to home in on wounded animals, whereas mammalian prey species avoid the same odour. This suggests that there may be an old, preserved, evolutionarily food and alarm molecule within the blood odour mixture that is the signal of blood. Researchers from Radboud University report in Scientific Reports of 20 October that they may have found this molecule called E2D, and it seems to affect humans as well.