I think I understand where you're coming from. I had a blood test done recently and it felt completely different on 'the other side of the needle' as it were. It was a bit like 'oh, am I really going to let this perfectly nice person stick a piece of metal into my arm...', but then I'm quite used to ignoring thoughts like that. It's just a bit incongruous- as I'm sure no where in our evolution was it ever advantageous until now to have stuff injected into you.
On another note. Everywhere I've tried in Manchester- even travel clinics have refused to vaccinate me. Though my GP were very nice and did blood tests, forms and letters explaining exactly why. Apparently they're not trained to inform me about other blood disease risks and stuff. But Cambridge have said that it's okay and they'll vaccinate me when I get there.