After syphilis, why can I not give blood?

Just as if you have a cut and get a scar, having syphilis will also leave a scar in your blood. This scar will be in the form of antibodies that won't disappear, so your blood screenings will always come up positive. If you are keen to give blood, you should probably go along to your local health clinic and ask for a test for syphilis. If you actually had another disease (such as gonnhorea) that leaves no trace, you will be able to give blood, But you are unlikely to be able to give blood if syphilis antibodies are present. As to your penicillin allergy, some people don't know they are allergic to something until they get exposed to it. If you were slightly allergic to penicillin before, receiving it for syphilis will have made your body expert at reacting to it with the result that any time you now take penicillin, your immune system will go into overdrive and give you an allergic reaction.