How come you can be allergic to things?

Answer

Every person has an immune system. That can distinguish what is your own (your heart, your muscles, your lungs, etc…) and what is not (viruses, bacteria and other pathogens). By making that distinction, your immune system can defend you against invaders. This prevents you from getting sick or from getting certain diseases such as rubella and measles twice.

For reasons that we do not yet fully understand (heredity and environmental pollution have to do with it), your immune system can react too strongly to substances in the environment (grass and tree seed, house dust, cat and dog hair,…). As a result, the tissues that come into contact with those substances also react too strongly. This causes eczema, asthma, hay fever and the like.