An old study in guinea pigs showed that hypersensitivity to fleas in that species occurred in 5 sequential stages. This orderly sequence does not occur in dogs. Animals that are continually exposed fail to develop hypersensitivity, while dogs that are intermittently exposed to fleas are the best candidates to develop flea allergy. Immunology studies on dogs constantly exposed to fleas found that these dots were not allergic. These studies suggest that these dogs developed tolerance.
Atopic dogs are more prone to become allergic to fleas.
Most flea allergic dogs have both an immediate and a delayed skin reactivity. There is a lot of variation among dogs. In general edema and eosinophils will persist in the area of a fleabite for 24-48 hours. The primary lesion seen with allergy is a papule. Which will appear 15 minutes after the bite and persist for up to 48-72 hours.