How can having more than one antibiotic allergy affect someone?

I’m hoping a medical person here will be able to chime in on this, as well as any Jellies who are allergic to more than one type of antibiotic. My son, who is now seven, is allergic to penicllin. We’ve known this since he was an infant, and it hasn’t been such a big deal.

Well, he’s been on Zithromax since yesterday for strep throat and tonight broke out in hives all over his back. They were spreading until the Benedryl kicked in. The doctor thinks it was probably the Zithromax, and tomorrow they’ll call in a prescription for something else.

Is there any serious down side to being allergic to more than one antibiotic?