What really causes allergic reactions is your immune system. It mistakes innocent things in your surroundings for a serious threat and attacks them. The symptoms you get are the result. How you get allergies is in your genes. You don't get a specific allergy but your tendency to get an allergy. Children with both parents who have allergies have a 70% of developing an allergy.
Since you can be allergic to many different thing the symptoms vary. When you have a food allergy you can get swelling of lips, throat, or lung. You can also hives itching eczema dizziness, light headed fainting If you are allergic to pollen in the different seasons you sneeze your eyes start watering and your nose gets congestion. Drug allergies can get skin rashes hives anaphlaxis (throat closes up, trouble breathing) or anemia.
The best way to prevent your allergies is to stay away fro them. Try your best to look at labels on food and drugs to see if what you're allergic to is in there. Also if your allergic to pollen or seasonal things make sure to look for the pollen levels and stay inside on very dry and hidden days.