Why are some people allergic to substances other people are not allergic to?

everybody's got different troubles with they're noses.like for example:you're allergic to bee stings,while somebodys allergic to they're pets fur.it's what's in the smell they breathe in.most people have atleast one thing they're allergic to,but some people haven't found anything,like me.sometimes it's mild like sneezing over and over again,and somtimes you take a trip to the ER.....