I'm a regular donor and have never heard of any problem with allergies. Because of that, I googled it and according to both SNBTS and the system in England (and Wales?) on a quick look, there shouldn't be a problem if you are not suffering from active allergic symptoms on the day you donate. If you are keen to get back to donating, it might be worth phoning whichever organisation is relevant to you, as the rules do keep changing. We don't get these texts in Scotland about where the blood has gone, but it sounds like a nice idea. I only do it for the cheesy biscuits, which I never get at any other time.