Why are bite reactions to ectoparasites so much different in different people?

The severity of bug bite reactions depends most on the immunologic status of the patient. In general, a severely symptomatic eruption may result from only a few bites in an immunologically sensitized person, while many bites may produce no symptoms at all in a person with an acquired immunologic tolerance.

In clinical settings and in experimental models, continuous exposure to biting arthropods will produce five stages of reactivity. Confusing clinical reactions may emerge because the immunologic reaction may range from nonreactive to severe and even tolerant, depending on the patient’s immune status.

• Stage 1: Bite is nonreactive in persons never exposed before.
• Stage 2: After a 2-week period of exposure, a delayed red papule begins within 24 hours after a bite and subsides within a week.
• Stage 3: An immediate wheal after a bite, followed by a delayed papule 24 hours later, becomes the clinical presentation.
• Stage 4: Immediate wheal develops as usual but the delayed hypersensitivity is lost.
• Stage 5: With continual exposure, immunologic tolerance begins, and there is no reaction after even prolonged exposure to insect bites; that is, with the immediate as well as the delayed response is lost.

Dahl MV: Cutaneous reactions to arthropod bites and stings, Clin Cases Dermatol 3:11–16, 1991.