If you take an antihistamine before you ate such as zyrtec, would that help prevent a severe allergic reaction such as anaphylaxis?

No: and better not to take any antihistamine prophylactically to prevent possibe severe allergic reaction to any suspected food that would cause allergy, what happens is that the antihistamine masks the initial warning signs that a more severe reaction is coming. You may want to keep an epinephrine injection handy, and avoid suspicious foods. Follow up with an allergist, check aaaai.org 4 an allergist