Guinea pigs acquired resistance to Dermacentor andersoni larvae. Administration of the
immunosuppressant, Methotrexate, prevented the acquisition of resistance. Resistant hosts
allowed relatively very few larvae to engorge, and showed marked infiltration of the infested
skin by basophil leucocytes. Particular high concentrations of basophils occurred in vesicles
in the hyperplastic epidermis beneath the mouth-parts of attached larvae. These changes
appear similar to Cutaneous Basophil Hypersensitivity reactions.