Your roommate is rushed to the hospital after suffering a severe allergic reaction to a bee sting. After she is treated and released, she asks you (the local biology expert!) to explain what happened. She says, 'I don't understand how this could have happened. I've been stung by bees before and didn't have a reaction.' Suggest an explanation for what has happened to cause her severe allergic reaction and why she did not have the reaction after previous bee stings.