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Professional antigen-presenting cells scour your dog’s skin looking for foreign substance like pollen, bacteria, dust mites or fungi. Once they find something suspicious, they present it to naive or uncommitted CD4+ T cells. In allergic dogs, those naïve T cells (a kind of lymphocyte) then become “committed(= “programmed” or “activated”) to produce IL-31 and other products that cause inflammation and itch. Some of those committed T cells hang around long after the offending substance is gone (memory T cells). They activate quickly when that foreign substance is again present. Those professional lookout or sentry cells are present in all your pet's body surfaces - even its lungs and intestine. So food allergens and inhaled allergens can potentially (probably not often) cause itching as well. The diagram will enlarge a bit if you click on it.