What can you do to treat an allergic reaction?

Depends on severity. Severe allergic reactions (like anaphylaxis) that involve chest/throat symptoms are treated with Epinephrine for immediate relief and oral steroids. Milder allergic reactions that affect skin, nose, eyes are usually treated with antihistamines. The real goal is to identify the trigger, so it can be avoided. An allergist is uniquely trained to diagnose and appropriately treat allergic reactions.