Anaphylactic and anaphylactoid reactions are severe allergic reactions that can be life-threatening. Anaphylactic reactions are caused by immunoglobulin E or G antibodies responding to a previously encountered antigen. Anaphylactoid reactions have similar symptoms but are caused by non-antibody related mechanisms during first exposure. Common allergens that can trigger these reactions include foods, insect stings, pollen, medications like antibiotics, and contrast agents used in imaging. Treatment involves stopping exposure, monitoring vital signs, compensating fluid loss, and administering epinephrine which constricts blood vessels and stimulates the heart.