Larry moved from chicago to atlanta. for the first two years he lived there he had no allergies in the spring. then, in the third year, he had very bad allergic reactions to spring pollens. what is the most likely explanation for this delayed effect?

This might have been the cause of not being exposed to this specific pollen during his years in Chicago. Allergic reactions can develop when the stimuli presented even when its not contagious causes the immune system to trigger and act in a defensive stance. This delayed effect may have been the cause of the sudden expression of the immune system. 

Ther are many reasons.. is it a multiple choice?