Can someone explain to me how come it is that certain people are allergic to some things and others are not?

Genetics: Some people have an immune system that is more likely to react incorrectly to allergens and when they are exposed to the allergen in the right way under the right conditions, they develop an antibody called IgE to that allergen. People who have a family history of allergy or few childhood infections, or less exposure to animals early in life are more likely to have this.