There are a lot of processes I skipped over. One of those is how the body removes B-Cells that react to benign or helpful proteins before they mature. But yes, if you had a B-Cell that produces antibodies against some antigen in Chicken soup, your scenario could lead to an allergy. The key is the body tries to select B-Cells that aren't going to produce antibodies that will be harmful to you. Sometimes it misses one and those are the ones that will cause an allergy or autoimmune disorder.