I get serum sickness from bee stings. So my worst reaction is about a week later. The area will swell up and cause a lot of discomfort. I’ve read a lot about it, because I was worried that the next sting would cause anaphylaxis. I got an Epi pen from my doctor, just in case. There doesn’t have to be a progression from serum sickness to more severe allergy reactions, but I want to be safe.