If I eat something I'm allergic to, what does it do to my body?

Depends on your normal reaction as allergies vary.
You could experience any number of symptoms, mostly covered here: Allergic Reaction Causes, Symptoms, Treatment - Allergic Reaction Symptoms and Signs on eMedicineHealth (http://www.emedicinehealth.com/allergic_reaction/page3_em.htm#allergic_reaction_symptoms_and_signs)