 Could alien blood be toxic? Let's face it, if we were to ever have an extraterrestrial encounter, it's the sort of question that could mean the difference between galactic peace and prosperity on one hand and a rather unfortunate interspecies faux pas on the other. Having a guest's nosebleed strip the flesh from your bones alien style probably isn't the best way to cement a lifelong relationship. Of course, despite Hollywood fantasies and the rapidly growing number of exoplanets being discovered, the chances of an alien encounter are remote. But nevertheless, it's interesting to speculate on how radically different biologies might react when they come together. Terrestrial biology is built around carbon and the complex molecule dioxyribonucleic acid or DNA. But this could just be an oddity of Earth. What if our extraterrestrial neighbours got lumbered with a biology based on another material – silicon for instance. It's not that likely as the chemistry of silicon is rather more limited than that of carbon but imagine for a second that it could happen. Silicon analogues of carbon-based molecules tend to be extremely reactive. This could be bad news for anyone inadvertently sharing bodily fluids with a silicon-based buddy. Forget about the blood, even a casual handshake could be fatal. To truth be told, our blood will probably be just as toxic to them as theirs was to us. Probably more so. Even if we stumble across another carbon-based life form, we may face problems. We've evolved to breathe and burn oxygen for energy. But what if our carbon-based neighbours breathed something else? We already know of terrestrial bacteria that use alternatives to oxygen for respiration including substances like iron, manganese, chlorine, sulphur and even uranium. If our new intergalactic friend was a bit of a sulphur snorter we could conceivably have a problem as rather than them excelling carbon dioxide it's more likely to be sulphuric acid, giving a whole new meaning to the irritating house guest syndrome. But what if we meet aliens who are almost but not quite like us? What if they're carbon-based oxygen-breathing beings based on something that looks similar to DNA? Could this not quite DNA cause us problems? Would our bodies know what to do with the alien nucleic acids it contained? The chances are that our protective enzymes would break down the foreign molecules and handle any potential hazards, but there could be lingering questions over immune responses as our bodies respond to something that they've never before encountered. This raises the tricky possibility of intergalactic species allergies. Not that we're likely to see diplomatic protocols for avoiding embarrassing alien rashes in our lifetime, but this last issue isn't as daft as it sounds. Scientists are already playing around with new molecules that might conceivably form the basis of totally artificial life forms. This synthetic biology is cutting-edge stuff but not beyond the bounds of possibility. So while we are highly unlikely to come face-to-face with true extraterrestrial aliens, given the speed with which science and technology are now progressing, it could be only a matter of time before we make our own homegrown aliens. At which point, asking whether alien blood could be toxic won't be quite as silly as you might think. Don't forget to share your thoughts on the risks of alien blood, whether extraterrestrial or homegrown, in the comments below. And stay safe.