The Summation Effect
The point at which a pet begins to itch and scratch indicates a summation of events, each of which contributes independently to the cascade of itchy discomfort that the pet is experiencing. This is called the pruritic threshold, or clinical threshold.

Pruritic thresholds are uniquely different in each animal. The summation effect explains why an animal who is allergic with low-level dermatitis to one or two mold allergens can appear to be completely normal, until she develops a new allergy to fleas. 

Taken separately, the flea or mold allergy may not cause her enough discomfort to “scratch,” but the summation effect of the two allergies together may cause her to reach a clinical threshold, where her discomfort manifests in clinically noticeable symptoms; she begins scratching.