text: will a food allergy in a toddler show itself immediately or is it possible it could take days? my 3 year old has developed welts all over his body, and he started a fever last night. an allergic reaction will not take days period.  it will begin when the body starts to metabolize the food.  much in the same way food poisoning shows up.  it can range from an hour to several, but not days later.  the food would already be processed & gone from the body in days.  same thing as contact dermititis.  it's not gonna be delayed days.