Hi! basically serum sickness is immune complex mediated type 3 hypersensitivity reaction caused by exposure to protein of nonhuman animal sources occuring 4 to seven days after exposure. Main agents causing this reaction are vaccine, antitoxin, and several drugs like penicillin, sulphonamide, quinidine etc. This reaction may present as rashes, joint pain, fever, abdominal pain, lymphadenopathy, malaise, itching, shock if further exposure occur. So try to prevent further exposure to the offending agents, for the management of present symptoms corticosteroids, antihistaminics, analgesics or plasmapheresis if the symptoms are not responding to medical treatment. As you are worrying about food allergy and atopy to environmental agents the I want to tell you that it is type 1 hypersensitivity reaction mediated by Ig E immunoglobulin and it can be diagnosed by clinical examination and family history, it has genetic basis of occurence but most commonly from maternal side.food causing allergy most commonly in these patient are peanuts, milk, egg, fish, shelfish, soya, wheat etc. but you can get the complete list by going for allergy panel test and then you can avoid exposure to all allergents. You can keep epinephrine injections for emergency anaphylactic reactions. Hope your child will be fine very soon.